Chemotherapy of bacterial infections. Part II. Mechanisms of Resistance. evolution of antimicrobial resistance

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1 Chemotherapy of bacterial infections. Part II. Mechanisms of Resistance evolution of antimicrobial resistance Mechanism of bacterial genetic variability Point mutations may occur in a nucleotide base pair, which is reffered to as microevolutionary change. These mutatuins may alter the target site of an antimicrobial agent, interfering with its activity. Macroevolutionary change results in rearrangements of large segments of DNA as a single event. May include inversions, duplications, insertions, deletions etc. Mechanism of bacterial genetic variability Acquisition of foreign DNA carried by plasmids, bacteriophages or transposable genetic elements. These foreign elements give the organism the ability to adapt to antimicrobial activity. Origin of drug resistance: intrinsic - inherent, natural acquired nongenetic - lack of active replication - loss of the specific target structure (L-forms) - infection ocurring at sites where antimicrobials are excluded or not active Natural resistance Is a stable characteristic of all strains of the same bacterial species Knowledge of natural resistances enables the inactivity of a molecule to be predicted in relation to the identified (after collection) or probable (in cases of empiric therapy) bacteria. It sometimes constitutes an identification aid since some species can be characterized by their natural resistance.

2 E.g. natural resistance of Proteus mirabilis to tetracyclines and colistin. natural resistance of Klebsiella pneumoniae to aminopenicillins (ampicillin, amoxicillin) Examples of natural resistance The genetic mechanism of acquired resistance can be: The mutation implied in the mode of action of the antibiotic.this mechanism mainly concerns the following antibiotics: quinolones, rifampin, fusidic acid, fosfomycin, antituberculosis drugs, sometimes cephalosporins. Example: reistance to quinolones by modification of DNA gyrase in Enterobacteriaceae. Acquisition of resistance genes transferred from a strain belonging to an identical or different species. Some antibiotics are particularly concerned by this mechanism : ß-lactams, aminoglicosides, tertracyclines, chloramphenicol, sulfonamides. Example: resistance to ampicillin in E.coli and Proteus mirabilis Efflux mechanism: expulsion of the molecule by active transport. Example: tetracycline-resistant staphylococci. ß-lactamase classes Major ESBL classes

3 Detection of ESBLs double disc test Discs are: left cefotaxime 30 µg; Centre µg amoxicillin/clavulanate Right ceftazidime 30 µg Placed 20 mm apart on agar plate Synergy is seen as an expansion of the cephalosporin zone adjacent to the clavulanate containing disc. Detection of ESBL producers by Etest The strips have cefotaxime alone at the end to the top of the ilustration and cefotaxime + clavulanate and the lower end. MICs that are 8-fold lower with the clavulanate, or keyhole zones as in the lower panel, imply ESBL production. The method is accurate and internally controlled expensive The selection risk is minimal for those beta-lactams that induce AmpC enzymes strongly e.g. carbapenems. AmpC beta-lactamases class C ß-lactamases carbapenems should be preferred over third-gen. cephalosporins for severe infection due to AmpC inducible species (Enterobacter and Citrobacter). Alternatives include fourth generation cephalosporins (cefepime and cefpirome) Temocillin an AmpC stable penicillin, not widely available AmpC beta-lactamases class C ß-lactamases Most AmpC enzymes fail to bind available ß-lactamase inhibitors, leaving derepressed strains resistant to inhibitor combinations, except than derepressed M.morganii are characteristically susceptible to piperacillin/tazobactam.

4 Once selected in one patient, AmpC-derepressed strains may spread to others. In the USA, in much of Europe and East Asia, over 30% of E.cloacae and C. freundii isolates are derepressed and resistant to oxyimino-cephalosporins at first isolation Genes for AmpC ß lactamases are escaping to plasmids that are spreading into E.coli and Klebsiella Plasmid-mediated AmpC enzymes are less prevalent than ESBLs but they are locally frequent and increasing. KPC class A KPC - Klebsiella pneumoniae carbapenemase producing by K.pneumoniae and other species of Enterobacteriaceae hydrolyze carbapenems Detection in lab: combined disc test using meropenem with and without PBA (boronic acid compounds as beta-lactamase inhibitors) The accurate detection of KPC- possesing Enterobacteriaceae is crucial in controlling their spread. S. pneumoniae; mosaic pbp genes (different level of β-lactams susceptibility) For over 10 years, pneumococcal resistance to penicillin G has continously increased in many countries (less than 1% in 1985, 10-50% in 1998) Reistance is generally associated with resistance to other antibiotics (tetracyclines, macrolides, cotrimoxazole..) ENT (ear, nose, throat), bronchopulmonary infections, meningitis are often caused by S.pneumoniae Resistance to penicillin G is extended to all beta-lactams, with different levels of resistance depending on the molecules of this family This requires futher testing to be performed (e.g. exact determinationof the MIC for penicillin G, amoxicillin, cefotaxime ) Macrolide-Lincosamide-Streptogramin Resistance

5 The MLS group: macrolides, lincosamides and streptogramins (A and B); resistance - due to efflux, target modification, drug enzymatic modification. In staphs, erythromycin resistance is caused by the erm gene coding an enzyme which modifies the ribosome NONE of these antibiotics may bind to the ribosome. If the erm gene product is constitutively produced, testing will find the staph resistant to all of the MLS antibiotics. If the erm gene product is inducibly produced, it will only be expressed when the organism is exposed to a macrolide C14 and C15, but not C16 and ketolides. Acquired Resistance to Aminoglycosides mainly enzymatic drug modification Kan HC Gen HC - HLAR E. faecalis Resistant Resistant No synergy with B-lactam and Kan or Gen Intermediate Intermediate Synergy with B-lactam and Kan or Gen Resistant Intermediate No synergy with B-lactam and Kan Glycopeptide resistance GISA, VISA aureus glycopeptide, vancomycin intermediate susceptible Staphylococcus Enterococci have natural susceptibility to vancomycin, except E. casseliflavus and E. galinarum (natural resistance to vancomycin). vana vanb vanc vana confers high level resistance to vancomycin vanb confers moderate level resistance to vancomycin vanc causes intrinsic vancomycin resistance in E. gallinarum and E. casseliflavus VRE (Vancomycin Resistant Enterococci) refers to E.fecalis or E.faecium, which are normally sensitive to vancomycin. Vancomycin resistance in these organisms is due to the acquisition of vana or vanb. Can antibiotics induce reistance?

6 Antibiotics do not induce resistance, but select resistant bacteria by eliminating susceptible bacteria known as selective pressure The increase in the frequency of resistant strains is most often linked to the intensive use of a specific antibiotic. Which methods enable resistance mechanism to be demonstrated in vitro? Direct detection of biochemical mechanism e.g. detection of ß-lactamase by hydrolysis of nitrocefin Direct detection of genetic determinants of resistance e.g. detection of the meca gene responsible for staphylococcal resistance to oxacillin Susceptibility test results can suggest the presence of a resistance mechanism. What is antibiotic equivalence? Equivalence is the prediction of antibiotic in vivo activity based on results obtained by testing another, related antibiotic. E.g. Equivalence between cephalotin which is tested and other 1 st generation cephalosporins which are not tested. The test result for the other molecules can be equated with that obtained for cephalotin. It is possible to test a restricted number of antibiotics without limiting therapeutic possibilities. Which criteria are used to select the antibiotics to be tested? The antibotics tested are those of therapeutic interest for the patohology considered and therefore, for the body site from which the specimen was collected Due to the importance of acquired resistance, it is also necessary to test antibiotics which have no therapeutic interest but constitute resistance markers -which are the most likely to detect resistance mechanism. E.g.. Oxacillin is an excellent penicillin resistance marker for S.pneumoniae. Cross resistance and associated resistance Cross resistance is a given resistance mechanism affecting a whole class of antibiotics.

7 E.g. For Streptococci resistance to macrolides can be predicted by testing erythromycin Resistance to oxacillin (cefoxitin) in Staphylococci is a cross resistance predicting in vivo resistance to all ß-lactams. In certain cases, it can affect antibiotics from different classes. E.g. resistance due to impermeability to cyclines with chloramphenicol and trimethoprim. Resistance are said to be associated when they concern antibiotics from different classes and when due to the association of several resistance mechanism. E.g.resistance to oxacillin is often associated with resistance to quinolones, aminoglycosides, macrolides and cyclines. Conclusion Antibiotic susceptibility testing, at the interface between the clinical diagnosis and the therapeutic decision, is a key element essential for guiding both microbiologically-documented and empiric antibiotic therapy. The evolution of bacterial resistance, as well as the development of new drug and laboratory techniques make a close working relationship between the microbiologist and the clinician more necessary now than ever before.