As antibiotic use increases in veterinary medicine, the issue of bacterial resistance

464 V Vol. 23, No. 5 May 2001 Email comments/questions to compendium@medimedia.com CE Article #5 (1.5 contact hours) Refereed Peer Review KEY FACTS Bacteria become resistant to antibiotics following a mutational event or by the acquisition of genes encoding a product that confers resistance. Although most experimentally derived bacterial resistance involves chromosomal changes, most natural resistance involves plasmid-mediated resistance factors. Through multidrug resistance mechanisms, organisms may acquire resistance to compounds to which they have not been exposed. Mechanisms of Antibiotic Resistance University of Wisconsin Stacey B. Hoffman, DVM, MA ABSTRACT: Emergence of increasing numbers of antibiotic-resistant pathogens in human medicine has raised concerns about antibiotic use in veterinary medicine. Bacteria have numerous mechanisms for exchange of genetic information, which allows acquired resistance to expand rapidly throughout a population. Mechanisms that confer antibiotic resistance to bacteria include exclusion of the drug from the internal environment, inactivation of the compound, or alteration of target proteins. Inappropriate antibiotic use serves only to enhance selection pressure for development of resistant bacterial strains. As antibiotic use increases in veterinary medicine, the issue of bacterial resistance to antimicrobial therapy becomes more worrisome. The emergence of increasing numbers of antibiotic-resistant pathogens has implications for not only veterinary patients but also humans. Many have speculated that antimicrobial use in veterinary medicine may contribute, directly or indirectly, to increased antimicrobial resistance in humans. 1,2 The use of subtherapeutic concentrations of antimicrobials as growth promoters in food animals and during treatment of companion animals has increased the potential reservoir of resistance genes in the environment. Resistant bacteria flourish in areas of high antimicrobial use since they have a selective advantage over the rest of the bacterial population. The issue of antimicrobial resistance is not a new one; in fact, it arose shortly after the introduction of antibiotics in the 1940s. 3,4 Historically, the introduction of new classes of antibiotics has ultimately progressed to the isolation of clinically resistant strains of bacteria to each new class. Strains of bacteria that are resistant to all available antimicrobials have recently been identified from clinical isolates of human patients. 3,5 Certain subclasses of antibiotics, such as the semisynthetic penicillins (oxacillin, methicillin), were developed specifically to counter resistance to original prototype compounds. 6 In addition to newer classes of older antibiotics, completely novel classes (e.g., glycopeptides, fluoroquinolones) have been developed. Understanding the mechanisms of antimicrobial action (Tables 1 and 2) is essential for understanding how antimicrobial resistance develops. RESISTANCE The antimicrobial susceptibility of an organism in vitro cannot be determined without accepted definitions of susceptibility and resistance. Most veterinary refer-

Compendium May 2001 Small Animal/Exotics 465 Antibiotic Class Inhibit growth by analogues Sulfonamides Inhibit cell wall synthesis Penicillins TABLE 1 Antimicrobials Available for Use in Small Animal Medicine Examples Trimethoprim sulfadiazine, ormetoprim sulfa Natural: penicillin G; semisynthetic: oxacillin, ampicillin, ticarcillin; potentiated: amoxicillin clavulanic acid Mechanism of Action Sulfonamides: competitive analogues of paraaminobenzoic acid required for folic-acid synthesis; trimethoprim: blocks folicacid formation by inhibiting dihydrofolate reductase Inhibit peptidoglycan synthesis necessary for cellwall formation Antimicrobial Activity Bacteriostatic; spectrum includes gramnegative bacteria; bactericidal with dihydrofolate reductase gene inhibitors Bactericidal; most active against gram-positive organisms; synthetic and potentiated penicillins have an improved gram-negative spectrum Cephalosporins First generation: cephalothin, cephalexin; second generation: cefoxitin, cefotetan; third generation: ceftriaxone, ceftazidime All generations act by inhibiting peptidoglycan synthesis necessary for cellwall formation Bactericidal; first generation: active against most gram-positive organisms with limited gram-negative activity; second generation: active against most gram-positive organisms with improved gram-negative coverage and some anaerobic coverage; third generation: limited gram-positive activity but excellent gram-negative and anaerobic spectrum Glycopeptides Inhibit protein synthesis Aminoglycosides Vancomycin Gentamicin, amikacin, tobramycin, neomycin Interact with bacterial cell membrane to prevent transfer of structural cell-wall units Bind 30S ribosomal subunit; inhibit peptide elongation Bactericidal; principally active against grampositive bacteria; used in emergencies to treat staphylococcal infections Bactericidal; active against gram-negative organisms, including Pseudomonas, Mycobacterium, and some gram-positive organisms Tetracyclines Tetracycline, doxycycline Bind 30S ribosomal subunit; inhibit trna function Bacteriostatic; gram-positive and gramnegative spectrum; also active against Rickettsia, Mycoplasma, and Chlamydia Chloramphenicol Chloramphenicol Bind 50S ribosomal subunit; inhibit protein synthesis Bacteriostatic; broad gram-positive and gram-negative spectrum Macrolides and lincosamides Inhibit nucleic acid synthesis Quinolones Erythromycin (macrolide), clindamycin (lincosamide) Enrofloxacin, ciprofloxacin, orbifloxacin Bind 50S ribosomal subunit; inhibit protein synthesis Inhibit DNA gyrase, preventing supercoiling and leading to DNA degradation Effective against gram-positive organisms as well as Mycoplasma, Chlamydia, and Rickettsia; clindamycin also has a good anaerobic spectrum Bactericidal; wide gram-positive and gram-negative spectrum, including Pseudomonas at a higher dosage

466 Small Animal/Exotics Compendium May 2001 Antibiotic Class Sulfonamides TABLE 2 Resistance Mechanisms for Antimicrobials Reported Mechanisms of Resistance Enzymatic bypass by synthesis of new druginsensitive enzymes Chromosomal Versus Plasmid-Mediated Predominantly plasmid-mediated (resistance factor), but mutants also reportedly occur Penicillins Cephalosporins Glycopeptides Aminoglycosides Tetracyclines Chloramphenicol Macrolides and lincosamides Quinolones Destruction of antibiotic by β-lactamases Failure of antibiotic to activate autolytic enzymes in cell wall Absence of penicillin receptors/alterations in penicillin-binding proteins Destruction of antibiotic by β-lactamases (first generation) Destruction of antibiotic by extendedspectrum β-lactamases (second and third generations) Production of altered, membrane-associated protein with decreased affinity for vancomycin Impaired transport of aminoglycoside across cell membrane; O 2 -dependent mechanism frequently affects anaerobes Inactivation of drug by one of three aminoglycoside-altering enzymes Impaired ribosomal binding Synthesis of cytoplasmic membrane efflux protein, which pumps drug out of the cell Formation of protein, which protects ribosome from tetracycline inhibition Energy-dependent inactivation of tetracycline Destruction of the drug by chloramphenicol acetyltransferase Alteration of the ribosomal receptor site New membrane-transport system causing active efflux Modification of DNA gyrase Failure of the drug to enter the organism due to diminished permeability Development of active efflux pump Predominantly plasmid-mediated but may be chromosomal Chromosomal mutation Predominantly plasmid-mediated but may be chromosomal Chromosomal mutations of plasmids to allow larger molecules into active site Transposon predominantly carried by selftransferrable plasmids but may be chromosomal Intrinsic property of anaerobes may be seen as chromosomal mutation Usually plasmid-mediated but may be chromosomal Minor resistance mechanism due to chromosomal mutation Several different proteins identified; all encoded on plasmids Chromosomal or plasmid-mediated; usually associated with conjugative transposon Plasmid-mediated Predominantly plasmid-mediated, but chromosomal mutations also reportedly occur Chromosomal mutation in gram-positive organisms Plasmid-mediated mechanism in gramnegative organisms Infrequently recognized chromosomal mutation Infrequently recognized chromosomal mutation in gram-negative bacteria Infrequently recognized chromosomal mutation

Compendium May 2001 Small Animal/Exotics 467 ence laboratories base their antibiograms on the minimum inhibitory concentration (MIC) of the organism. 7 9 The MIC is the minimum concentration of a drug needed to inhibit bacterial growth under laboratory conditions. The breakpoint of an antimicrobial is the plasma concentration above which the host is likely to experience toxicity 7 and is dependent on the pharmacokinetics of the antibiotic in the host, not the bacterial isolate itself. 2,8 If the MIC of an organism is lower than the breakpoint MIC, the organism is considered to be susceptible to the antimicrobial. While MIC may be used to guide antimicrobial therapy in patients, results of in vitro tests may not always translate into clinical response in patients. To be effective, a drug must be able to reach active concentrations at least equal to the MIC at the site of infection. 7,8 MIC breakpoints are based on plasma concentrations of antimicrobials and may not reflect drug concentrations at the site of infection. For example, such obligate intracellular pathogens as Rickettsia are susceptible in vivo only to lipid-soluble drugs, which can diffuse through the cell membrane. In addition, such local tissue factors as low ph and necrotic material may render an antimicrobial less effective than laboratory tests would predict, causing a phenomenon known as pseudosensitivity. 10 Conversely, in vitro tests may suggest a lack of response to antimicrobials that may actually be effective in vivo. For example, urinary concentrations of a fluoroquinolone or β-lactam may be much higher than plasma concentrations, resulting in greater efficacy for urinary tract infections in patients than in vitro testing would predict. Antimicrobial agents have traditionally been classified as either bacteriostatic or bactericidal based on their mechanism of action. 7,11 The distinction between bactericidal and bacteriostatic antibiotics is fluid, depending on the pathogen involved and the achievable drug concentration at the site of infection. The MIC is the amount of drug necessary to inhibit microbial growth in vitro without killing the organism. If a drug inhibits growth of an organism by bacteriostatic (as opposed to bactericidal) mechanisms, the host must have adequate immunologic defenses to eliminate the infection. Immunocompromised patients, such as neonates and those receiving chemotherapy, may require bactericidal antibiotics to eliminate infection. Since MIC alone cannot account for variations between in vitro culture data and host defenses, it is often multiplied by a therapeutic factor to determine a target plasma drug concentration. 7,12 For treatment, target plasma drug concentrations are generally at least four times the MIC. INTRINSIC VERSUS ACQUIRED RESISTANCE Bacterial resistance to antimicrobials may be either intrinsic or acquired. Intrinsic Resistance Mechanisms Intrinsic resistance mechanisms are inherited properties of a given species of bacteria that are based on a lack of either antimicrobial target sites or accessibility to them. 13 For example, obligate anaerobes are considered intrinsically resistant to all aminoglycosides since they lack the electron transport system necessary for drug uptake. 14 Many gram-negative organisms, including Pseudomonas, are intrinsically resistant to macrolides and certain β-lactam antibiotics because the drugs are too hydrophobic to diffuse through the outer bacterial membrane. 15 Acquired Forms of Resistance Of greater concern are acquired forms of resistance that arise in response to selection pressures on a bacterial population. Genes encoding antibiotic resistance are often present in a population before exposure to a particular drug. While antimicrobial drugs do not typically cause genetic mutations, their presence creates a pressure for selection of mutants. To date, acquired forms of resistance have been identified in every pathogenic bacterial genus as well as in numerous types of commensal flora. 16 For a single organism to express resistance, it must change its genetic makeup by alteration of its DNA. Spontaneous chromosomal point mutations of DNA occur at a frequency of approximately 1 in 10 8, depending on environmental conditions. Although they arise infrequently, chromosomal mutations can be passed on to daughter progeny with a high degree of fidelity. Chromosomal mutations that contribute to antimicrobial resistance tend to produce modifications in the antibiotic target, such as changes in the bacterial cell wall. 11,16 Although most antibiotic-resistant strains of bacteria isolated under laboratory conditions involve chromosomal mutations, these mutations are seen less frequently in vivo. Many of these chromosomal mutations actually make the bacteria more sensitive to destruction by host defenses. Low-dose intermittent antibiotic administration creates an environment that favors the development of chromosomal resistance. 16 The genetic information in most bacteria consists of both chromosomal DNA and extrachromosomal DNA in the form of plasmids. Plasmids are small circular pieces of double-stranded DNA, which exist in a supercoiled state distinct from the chromosome. 11 Although they do not carry genes for essential metabolic activities, they may confer accessory properties such as virulence or antibiotic resistance. 17 Plasmid-based, antibiotic-resistance genes are referred to as resistance (R) factors. The majority of drug-resistant strains of bacteria isolated from human and veterinary patients involve

468 Small Animal/Exotics Compendium May 2001 R factors. 11 Although they have been found in every bacterial species, R factors are most common among Enterobacteriaceae. Conjugative transposons are another type of mobile genetic element associated with the development of antibiotic resistance. Transposons can excise themselves from the donor chromosome and insert themselves into recipient chromosomes or plasmids. EXCHANGE OF GENETIC INFORMATION Once a single organism has successfully acquired antimicrobial resistance, it must rapidly spread it to the remainder of the population. The ability to exchange genetic information coupled with short doubling times creates the potential for rapid development of resistant pathogens. Donor bacteria have only three mechanisms for the transfer of genetic information to a recipient: chromosomal, plasmid, or transposon. 11,16 Transformation Transformation is the uptake of naked DNA from a lysed donor bacterium directly into a competent bacterium. Since it is an inefficient means of genetic transfer with less than 1% of cells acquiring new genetic information, it is probably not a significant mechanism in the development of antibiotic resistance. Transduction Transduction is the transfer of genetic material using bacteriophages, viruses that multiply within bacteria. The greatest clinical significance of transduction in veterinary medicine is as a mechanism for transferring virulence genes, and it is only rarely involved in the transfer of antibiotic resistance genes. 16,18 Conjugation The most efficient means of DNA transfer between bacteria involves conjugation, a mating process that requires cell-to-cell contact. 11 A copy of a plasmid or transposon is transferred from donor to recipient, donating genetic information that may include antibiotic resistance. The process is so efficient that virtually every cell that forms a conjugative pair acquires new genetic information. Plasmid-mediated resistance may be passed both horizontally by conjugation and vertically at replication since daughter cells contain copies of the plasmid. Conjugation typically occurs between bacterial strains of the same species or across species within a family, such as between Escherichia coli and Salmonella. Transfer of genetic information between gram-negative and gram-positive organisms has been reported in vitro, particularly in Staphylococcus aureus. With the exception of staphylococci, however, most gram-positive organisms lack the ability to acquire R factors. 11,19 Intergeneric transfer events are of particular importance in areas that contain mixed bacterial populations (e.g., gastrointestinal tract). Nonpathogenic bacteria among the normal flora of humans and animals are capable of acquiring antimicrobial drug resistance. Ubiquitous, less-pathogenic organisms (e.g., enterococci) may then serve as environmental reservoirs for antibiotic resistance genes. 19 MECHANISMS OF ANTIMICROBIAL RESISTANCE Regardless of the mechanism by which an organism acquires antimicrobial resistance, there are only a few basic mechanistic categories of resistance. The first mechanism by which resistance can occur is by exclusion of the antimicrobial from the internal environment of the bacterium. A target antibiotic is rendered inactive if it cannot reach appropriate concentrations within the organism. The second general mechanism of antibiotic resistance involves inactivation of the antimicrobial compound itself. The last mechanism of resistance involves altering the antibiotic target sites, which are often part of the cellular replication pathway. Not every resistance mechanism will give rise to clinically significant or detectable resistance. 20 A summary of resistance mechanisms reported for various antimicrobials is listed in Table 2. Exclusion from the Internal Environment The outer membrane of bacteria serves as a protective barrier from the environment, and alterations in its permeability can confer antibiotic resistance. This resistance mechanism is encountered primarily in gramnegative bacteria. Intrinsic antibiotic resistance due to poor outer-membrane permeability has been reported in Pseudomonas aeruginosa and Bacteroides fragilis. Nonspecific protein channels that span the outer membrane are known as porins. Porins may exclude penetration of large or hydrophobic compounds, and antimicrobial resistance has been reported due to loss or modification of certain porins. 15,21 23 Another method to prevent high intracellular concentrations of antibiotic is to remove the antibiotic at a rate that exceeds its uptake. The best-studied example of this type of resistance mechanism is the tetracycline efflux pump. 22 This plasmid-based gene encodes a cytoplasmic membrane protein that acts as an energy-dependent tetracycline transporter. 11,22 The exact mechanism by which this protein pumps tetracycline out of cells is not known. 22 An active efflux mechanism that pumps hydrophilic fluoroquinolones and other antibiotics out of gram-negative bacteria has also been reported. 20

470 Small Animal/Exotics Compendium May 2001 Inactivation of Antibiotics Another resistance mechanism used by bacteria involves inactivation of the parent antibiotic compound. Plasmids that produce antibiotic-modifying enzymes and confer resistance to aminoglycosides and chloramphenicol have been identified. These enzymes render the antibiotics inactive by selective acetylation or phosphorylation, respectively. 19 The most widespread example of antibiotic inactivation is the β-lactamases, a class of enzymes that hydrolyze the β-lactam nucleus of such antibiotics as penicillins and cephalosporins. β-lactamases are produced by numerous gram-negative organisms, some gram-positive organisms, and even such anaerobes as Bacteroides. 14,24 The genes for most β- lactamases are encoded on plasmids. β-lactamase producing plasmids were first identified in the 1960s and are now considered ubiquitous in gram-negative organisms. Newer generations of cephalosporins and β-lactams have been developed to counter antibiotic resistance to β- lactamases. The use of these second- and third-generation cephalosporins in veterinary medicine is typically limited to resistant gram-negative organisms due to their high cost and frequent dosing schedules. Extended-spectrum β-lactamases (ESBLs) that confer resistance to these newer generations of cephalosporins have been identified in E. coli, Enterobacter, and Klebsiella. 25 ESBLs have arisen through mutations in β-lactamase plasmids, which allow them to accommodate the bulkier side chains of newer cephalosporins. The heavy use of such newer-generation cephalosporins as ceftazidime in human hospitals has been associated with nosocomial outbreaks of ESBL-producing organisms. 25 The significance of ESBLs in veterinary medicine has not yet been evaluated. Altering Antibiotic Targets The targets of β-lactam antibiotics are cell-wall structural enzymes known as penicillin-binding proteins (PBPs). PBPs function in the terminal stages of cellwall synthesis and are essential for bacterial growth. If a β-lactam antibiotic is able to compete for the active site of a PBP, cell death occurs. PBPs are present in almost all bacteria, with variation among species in terms of their number, size, and antibiotic affinity. 4,24 Methicillin is a semisynthetic penicillin that was developed to counter β-lactamase production in S. aureus. Methicillin resistance in strains of S. aureus is due to a chromosomal mutation that leads to overproduction of an altered PBP 6,24 with low binding affinity for β-lactam antibiotics. In human medicine, nosocomial infections caused by methicillin-resistant S. aureus (MRSA) are of particular concern because they are often associated with resistance to a wide range of other antibiotics. 3,24 A recent case series identified 11 isolates of MRSA from dogs 26 ; 8 of these dogs developed MRSA infections as postsurgical complications, while the remaining 3 specimens were from pyodermas. All 11 of these isolates were susceptible to at least one orally available antibiotic, and therapy provided clinical improvement in 9 of 11 dogs. Another example of alteration of antibiotic targets includes chromosomal mutations in DNA gyrase, which is the primary target of fluoroquinolones. Various ribosomal protection mechanisms have been found to confer resistance to tetracyclines and macrolides, which depend on binding to bacterial ribosomes to inhibit protein synthesis. 14,22 MULTIDRUG RESISTANCE Approximately 50% of nosocomial infections in human hospitals are caused by organisms that exhibit multidrug resistance. 2 Almost all resistance plasmids confer resistance to more than a single antimicrobial because the segments of DNA they contain are so large (up to 150 kilobases in length). 6,18 The most common multiple resistance genes found on plasmids confer resistance to chloramphenicol, streptomycin, sulfonamides, and tetracyclines. Cross-resistance is also frequently encountered across entire classes of drugs so that development of resistance to one drug in a class confers resistance to others. 16,20 This is particularly true among tetracyclines, aminoglycosides, and fluoroquinolones. Multidrug resistance may also occur when one resistance mechanism affects the function of several unrelated antimicrobials, usually through changes in the cell wall. 6 Porin mutations in gram-negative bacteria may confer resistance to β-lactams, tetracyclines, chloramphenicol, sulfonamides, and fluoroquinolones. 15 Multidrug transporter systems, which may arise from deregulation of chromosomal genes governing flux of drugs across the bacterial cell wall, also result in resistance to multiple antimicrobials. 6 Obviously, pathogens with multiple antibiotic resistance pose a problem to the practitioner by limiting the number of antibiotics available for use. Multidrug resistance mechanisms may also allow a bacterial population to acquire resistance to an antibiotic to which it has not been exposed. Selection pressure for this multidrug resistance can be maintained by any one antimicrobial in the group. The selective pressure will end only if all antimicrobials to which the organism is resistant are withdrawn from the environment. 3 NOSOCOMIAL INFECTIONS Recent estimates place the number of nosocomial infections among patients in human hospitals at over two million/year. 27 Antibiotic pressure is the single most im-

Compendium May 2001 Small Animal/Exotics 471 portant factor in predisposing hospitalized human patients to infection with resistant organisms. 5 Resistant strains of bacteria most commonly emerge in geographic locations where antibiotic use is most frequent, such as large referral centers. Because these organisms often exhibit multidrug resistance, they are associated with high morbidity and mortality. Nosocomial infections have been reported in veterinary teaching hospitals, with the most common routes of exposure being the urinary tract, respiratory system, and wounds. 28,29 Acinetobacter baumannii, a normal commensal organism of the skin and genital tract of humans and animals, has been an opportunistic pathogen in human hospitals since the 1970s. A series of nosocomial A. baumannii infections in dogs and cats was recently reported from the intensive care unit of a veterinary teaching hospital. 30 In approximately half of the cases (9 of 17), the A. baumannii infection contributed to the death or euthanasia of the patient. Although the incidence of nosocomial infections in veterinary medicine is still relatively low, it will increase in importance as intensive care veterinary medicine continues to evolve. Many nosocomial infections arise from opportunistic infections originating with the patient s own microbial flora. Moreover, such resistant organisms as MRSA, which were once thought to arise only as nosocomial infections, have also been found as community-acquired infections in humans. Barrier nursing precautions, such as wearing gloves and handwashing after patient handling, have been shown to be the most effective measures in preventing the spread of nosocomial infections between patients. CONCLUSION The driving forces of emerging antimicrobial resistance are repeated exposure of the bacteria to antibiotics and access of bacteria to a large antimicrobial resistance gene pool. Increasing the number of mobile resistance elements in the gene pool of a population increases the likelihood that pathogenic organisms will acquire resistance genes. 18 In this way, resistance selected in harmless bacteria may ultimately cause problems in pathogens by acting as a reservoir of resistance genes. Resistant organisms may persist in the host or in the environment in the absence of antimicrobial selection. Large-scale agricultural surveillance programs have been instituted worldwide to evaluate the role of antibiotics as growth promoters on the development of resistant pathogens. The use of antibiotics in production medicine is frequently cited as a potential cause of antibiotic resistance in both human and veterinary medicine. 31,32 In Europe, the number of resistant Campylobacter and Salmonella strains isolated from humans increased following the approval of fluoroquinolones for use in food animals. 33 Concerns about the role of veterinary medicine in contributing to antibiotic resistance may lead to decreased veterinary access to future antimicrobials. New pathogen surveillance programs in veterinary medicine will provide data that may serve as an early warning when resistance to certain antibiotics is increasing. Ultimately, antimicrobial therapy should be designed with respect to organisms and patients as well as drugs. Optimal use of antibiotics is aimed at achieving maximum therapeutic effects with minimal selective pressure for resistance. When possible, culture and sensitivity data should be used to guide antibiotic selection, particularly in cases of resistant or recurrent infections. Since incorrect use of antibiotics may contribute to resistance, veterinarians and their clients must be aware of the potential risks and benefits of antibiotic administration. A study of resistance elements in enteric bacteria from humans and domestic pets in Australia found that their resistance plasmids were similar. This suggests a common origin for these R factors, with the potential for resistant organisms in one population to act as a source of infection for another. 34 Within hospital settings, the emphasis must remain on preventing nosocomial infections rather than relying on antibiotics as magic bullets. Increasing antibiotic resistance has the potential to move both human and veterinary medicine back to the preantibiotic era. REFERENCES 1. Piddock L: Does the use of antimicrobial agents in veterinary medicine and animal husbandry select antibiotic-resistant bacteria that infect man and compromise antimicrobial chemotherapy? J Antimicrob Chemother 38:1 3, 1996. 2. Brown SA: Clinical controversies: Does antimicrobial use cause a risk to man? Proc 16 th ACVIM Forum:174 175, 1998. 3. Tomasz A: Multiple antibiotic-resistant pathogenic bacteria. N Engl J Med 330:1247 1251, 1994. 4. Medeiros A: Evolution and dissemination of beta lactamases accelerated by generations of beta lactam antibiotics. Clin Infect Dis 24:S19 S45, 1997. 5. Tenover F, McGowan J: Reasons for the emergence of antibiotic resistance. Am J Med Sci 311:9 16, 1996. 6. Jacoby G, Archer G: New mechanisms of bacterial resistance to antimicrobial agents. N Engl J Med 324:601 611, 1991. 7. Boothe DM: MIC and PDC: What do they mean to me? Proc 14 th ACVIM Forum:393 395, 1996. 8. Whittem T: Empirical antimicrobial therapy. Proc 14 th ACVIM Forum:396 398, 1996. 9. Yancey JW: Identification of veterinary pathogens by use of commercial identification systems and new trends in antimicrobial susceptibility testing of veterinary pathogens. Clin Microbiol Rev 7:346 356, 1994. 10. Cunha B: Antibiotic resistance: Myths, truths, and a rational formulary approach. Formulary 34:664 682, 1999.

472 Small Animal/Exotics Compendium May 2001 11. Carter GR, Chengappa MM, Roberts AW: Essentials of Veterinary Microbiology. Baltimore, Williams and Wilkins, 1995, pp 1 38. 12. Dowling PM: Rational antibiotic therapy. Can Vet J 37:246 249, 1996. 13. Schwarz S, Werckenthin C, Pinter L, et al: Chloramphenicol resistance in Staphylococcus intermedius from a single veterinary centre: Evidence for plasmid and chromosomal location of the resistance genes. Vet Microbiol 43:151 159, 1995. 14. Rasmussen BA, Bush K, Tally FP: Antimicrobial resistance in anaerobes. Clin Infect Dis 24:S110 S120, 1997. 15. Nikaido H: Outer membrane barrier as a mechanism of antimicrobial resistance. Antimicrob Agents Chemother 33: 1831 1836, 1989. 16. Prescott J, Baggot JD: Antimicrobial Therapy in Veterinary Medicine. Ames, Iowa State University Press, 1993. 17. Schwarz S, Noble W: Aspects of bacterial resistance to antimicrobials used in veterinary dermatological practice. Vet Dermatol 10:163 176, 1999. 18. Boothe D: Drug therapy of the respiratory tract. Proc NAVC:6 17, 2000. 19. Leclercq R: Enterococci acquire new kinds of resistance. Clin Infect Dis 24:S80 S84, 1997. 20. Acar JF, Goldstein FW: Trends in bacterial resistance to fluoroquinolones. Clin Infect Dis 24:S67 S73, 1997. 21. Moosdeen F: The evolution of resistance to cephalosporins. Clin Infect Dis 24:487 493, 1997. 22. Speer B, Shoemaker N, Salyers A: Bacterial resistance to tetracycline: Mechanisms, transfer, and clinical significance. Clin Microbiol Rev 5:387 399, 1992. 23. Iaconis J, Pitkin D, Sheikh W, Nadler H: Comparison of antibacterial activities of meropenem and six other antimicrobials against Pseudomonas aeruginosa isolates from North American studies and clinical trials. Clin Infect Dis 24:S191 S196, 1997. 24. Georgopapadakou NH: Penicillin-binding proteins and bacterial resistance to beta-lactams. Antimicrob Agents Chemother 37:2045 2053, 1993. 25. Thomson K, Prevan A, Sanders C: Novel plasmid-mediated beta lactamases in Enterobacteriaceae: Emerging problems for new beta-lactam antibiotics. Curr Clin Top Infect Dis 151 163, 1996. 26. Tomlin J, Pead MJ, Lloyd DH, et al: Methicillin-resistant Staphylococcus aureus infections in 11 dogs. Vet Rec 144:60 64, 1999. 27. Williams R, Heymann D: Containment of antibiotic resistance. Science 279:1153 1154, 1998. 28. Wise L, Jones R, Reif J: Nosocomial canine urinary tract infections in a veterinary teaching hospital (1983 1988). JAAHA 26:148 152, 1990. 29. Lippert A, Fulton R, Parr A: Nosocomial infection surveillance in a small animal intensive care unit. JAAHA 24:627 636, 1988. 30. Francey T, Gaschen F, Nicolet J, Burnens A: The role of Acinetobacter baumannii as a nosocomial pathogen for dogs and cats in an intensive care unit. J Vet Intern Med 14:177 183, 2000. 31. Woodford N, Adebiyi A-M, Palepou M-F, Cookson B: Diversity of VanA glycopeptide resistance elements in enterococci from humans and nonhuman sources. Antimicrob Agents Chemother 42:502 508, 1998. 32. Hawkey PM: Action against antibiotic resistance: No time to lose. Lancet 351:1298 1299, 1998. 33. Tollefson L, Angulo F, Fedorka-Cray P: National surveillance for antibiotic resistance in zoonotic enteric pathogens. Vet Clin North Am Food Anim Pract 14:141 150, 1998. 34. Davies M, Stewart P: Transferable drug resistance in man and animals: Genetic relationship between R-plasmids in enteric bacteria from man and domestic pets. Aust Vet J 54:507 512, 1978. About the Author Dr. Hoffman is affiliated with the School of Veterinary Medicine, University of Wisconsin, Madison. ARTICLE #5 CE TEST The article you have read qualifies for 1.5 contact hours of Continuing Education Credit from the Auburn University College of Veterinary Medicine. Choose the best answer to each of the following questions; then mark your answers on the postage-paid envelope inserted in Compendium. 1. The MIC of an antibiotic is the a. plasma concentration above which host toxicity is seen. b. concentration that prevents bacterial growth in vitro. c. plasma concentration needed for clinical efficacy. d. lowest blood level that can be detected following drug administration. 2. Which of the following is not considered an acquired form of bacterial resistance? a. the resistance of anaerobic bacteria to aminoglycosides b. the energy-dependent tetracycline efflux pump c. multidrug transporter systems d. ESBLs 3. The most efficient means of transfer of resistance genes through a population is a. transformation. b. transposition. c. chromosomal mutations. d. conjugation. 4. Which of the following statements regarding plasmidbased resistance genes is true? a. Most antibiotic-resistant bacteria isolated from canine and human patients are associated with plasmid-based resistance genes. b. A single plasmid may confer resistance to several antibiotics. c. They may be transferred between bacteria by conjugation and to progeny at the time of replication. d. all of the above

Compendium May 2001 Small Animal/Exotics 473 5. Resistance caused by enzymatic inactivation of the antibiotic by bacteria has not been reported for a. fluoroquinolones. b. chloramphenicol. c. penicillins. d. aminoglycosides. 6. Which of the following mechanisms has not been shown to cause multidrug resistance? a. transfer of plasmids containing multiple resistance genes b. bacterial cell-wall alterations, which affect permeability c. inactivation of DNA gyrase d. multidrug transporter systems, which pump antibiotics out of bacteria 7. When local host factors render an antibiotic less effective than in vitro tests would predict, the phenomenon is known as a. intrinsic resistance. b. pseudosensitivity. c. acquired resistance. d. bacteriostatic effect. 8. Resistance to fluoroquinolones has not been reported to occur through which of the following mechanisms? a. modifications of DNA gyrase b. development of an active efflux pump c. mutations in porin channels d. acquisition of resistance plasmids 9. Enteric bacteria, which are not usually pathogenic, may a. acquire plasmid-based resistance genes through conjugation. b. serve as a reservoir of potential resistance genes for other organisms. c. be significant nosocomial pathogens for hospitalized patients. d. all of the above 10. Which of the following statements regarding β-lactamases is false? a. They are infrequently encountered in gram-negative organisms. b. They inactivate penicillins and cephalosporins. c. They are usually encoded on plasmids. d. Mutations may allow them to confer resistance to newer generations of cephalosporins.