Bacteria
Overview Bacteria live almost everywhere. Most are microscopic ranging from 0.5 5 m in size, and unicellular. They have a variety of shapes when viewed under a microscope, most commonly: Spheres, Rods, Spirals There are commonly found arrangements of bacteria based on their division.
Typical bacterial cell Capsule Flagella Fimbriae Bacterial DNA (Single chromosome) Ribosomes Plasma membrane Cell wall
Fig. 27-2 (a) Spherical (cocci) 1 µm 2 µm (b) Rod-shaped (bacilli) (c) Spiral 5 µm
Sphere Single coccus, plural cocci Diplococci in pairs Streptococci in chains Staphylococci in irregular clusters
Rod Single bacillus, plural bacilli Diplobacilli in pairs Streptobacilli in chains
Spiral These bacteria range in size from 1 m to over 100 m
Bacteria maintain their cell shape, and are protected from their physical environment by their cell wall. Bacterial cell walls contain a material called peptidoglycan.
Using the Gram stain, scientists classify many bacteria into Gram-positive and Gram-negative groups based on cell wall composition Gram-negative bacteria have less peptidoglycan and an outer membrane that can be toxic. Many antibiotics target peptidoglycan and damage bacterial cell walls, so Gram-negative bacteria are more likely to be antibiotic resistant.
Fig. 27-3 Cell wall Peptidoglycan layer Plasma membrane Carbohydrate portion of lipopolysaccharide Cell wall Outer membrane Peptidoglycan layer Plasma membrane Protein Protein (a) Gram-positive: peptidoglycan traps crystal violet. Grampositive bacteria Gramnegative bacteria 20 µm (b) Gram-negative: crystal violet is easily rinsed away, revealing red dye.
Fig. 27-3a Cell wall Peptidoglycan layer Plasma membrane Protein (a) Gram-positive: peptidoglycan traps crystal violet.
Fig. 27-3b Carbohydrate portion of lipopolysaccharide Cell wall Outer membrane Peptidoglycan layer Plasma membrane Protein (b) Gram-negative: crystal violet is easily rinsed away, revealing red dye.
Fig. 27-3c Grampositive bacteria Gramnegative bacteria 20 µm
A polysaccharide or protein layer called a capsule covers many bacteria.
Fig. 27-5 Some bacteria have fimbriae or attachment pili which allow them to stick to surfaces or other bacteria in a colony. 200 nm Fimbriae
Bacteria are motile, meaning they can move around. Most bacteria use flagella to propel themselves.
Bacteria are prokaryotes, pro- meaning before and kary- meaning nucleus. They do not have compartments or membrane bound organelles. Their genome is made of a single circular chromosome. Some species of bacteria also have smaller rings of DNA called plasmids.
Bacteria reproduce quickly by binary fission and can divide every 1-3 hours. Many bacteria can also form inactive endospores, which means they can remain viable in harsh conditions for centuries. Endospores form when the environment is too harsh.
Fig. 27-9 Endospore 0.3 µm
Even though bacteria reproduce asexually, they have an immense amount of genetic diversity. There are three factors that contribute to this genetic diversity: Rapid reproduction Mutation Genetic recombination
In a process called transformation, a bacterial cell can take in and incorporate foreign DNA from the surrounding environment. Conjugation is the process where genetic material is transferred between bacterial cells. Sex pili allow cells to connect to transfer DNA
Fig. 27-12 Sex pilus 1 µm
Plasmid Pilus Bacterium Donor Chromosome Recipient Plasmid is replicated New donor New donor
Many bacteria have plasmids that carry genes for antibiotic resistance. Antibiotics kill bacteria that aren t resistant, but the bacteria that are resistant survive, and have less competition. Antibiotic resistant strains of bacteria are becoming more common.
How bacteria cause disease Pathogenic bacteria typically cause disease by releasing exotoxins or endotoxins Exotoxins cause disease even if the bacteria that produce them are not present. Example botulinum toxin produced by Clostridium botulinum Botox
Exotoxins fall into three categories: Cytotoxins kill cells Neurotoxins interfere with normal nerve impulse - such as botulism Clostridium botulinum results in muscle paralysis - botox Enterotoxins affect cells lining the gastrointestinal tract Exotoxins can be inactivated by heat or chemicals and can no longer cause disease, but they can be used to create toxoids. Toxoids are injected to stimulate the production of antitoxins and provide immunity vaccine.
Endotoxins are released only when Gramnegative bacteria die and their cell walls break down Example Escherichia coli
Toxins are the primary factor in pathogenicity or ability to cause disease. There are 220 known bacterial toxins and almost half cause damage to the cell membrane. Bacteria have caused some of the most widespread epidemics in human civilization.
Bubonic Plague Bubonic plague is caused by the bacteria Yersinia pestis. It is a Gram-negative rodshaped coccobacillus. It is transmitted to humans from animals primarily by rats that are infected by fleas. The plague or bubonic form of infection is very severe with a fatality rate of 30 60 % if left untreated.
The name bubonic comes from the appearance of a bubo which is a swollen lymph node that can become an open sore as the infection advances. The bubonic plague that occurred in 14 th century Europe is one of the most destructive pandemics in history resulting in between 100 and 200 million deaths.
Antibiotics or antibacterials are drugs used to treat infections caused by bacteria. Antibiotics work in one of two ways: Bacteriocidal antibiotic kills bacteria Bacteriostatic stops bacteria from multiplying Many antibiotics are naturally occuring chemicals made by other microorganisms.
The first antibiotic was discovered in 1928 by Sir Alexander Fleming. He made a chance discovery while looking at agar plates he was going to throw away. He discovered a plate growing Staphylococcus aureus that had been contaminated. The mold contamination had killed the S. aureus. Penicillin works by targeting the bacterial cell wall, an causing it to rupture. Many antibiotics target key bacterial functions or ability to grow and divide.
Antibiotic resistance occurs when populations of bacteria stop being sensitive to an antibiotic. The evolution of resistant strains is a natural phenomenon that happens when microorganisms are exposed to antibiotics and resistant traits can be exchanged. However the misuse of antibiotics speeds up this natural process.