Cocci: diplococci (pairs), streptococci (chains), staphylococci (clusters — like grapes). Bacilli: single or chains. Spirochetes: flexible spirals (Treponema pallidum, Borrelia). Shape and arrangement help identify genus on smear.
Coccus
Round — Staph, Strep
Bacillus
Rod — E. coli, Bacillus
Spirillum
Spiral — Treponema, Campylobacter
Endospores
Endospores: heat-resistant dormant structures — Bacillus and Clostridium species
Endospores
Near-indestructible bacterial survival structures
Bacillus anthracis (anthrax) and Clostridium species (tetanus, botulism, C. diff) form endospores. Survive heat, drying, radiation, most disinfectants. Require autoclave (121°C, 15 min) to kill. Spores germinate when conditions improve.
Bacterial Growth Curve
Growth curve: Lag → Log → Stationary → Death. Antibiotics work best in Log phase.
Bacterial Growth Curve
Four phases of bacterial population growth
Lag: bacteria adjust to environment, no division yet. Log (exponential): rapid binary fission — doubling every generation. Stationary: nutrients depleted, growth = death rate. Death: population declines. E. coli generation time ~20 min; M. tuberculosis ~24 hours.
Bacterial Cell Wall
Peptidoglycan: NAM + NAG cross-linked by peptide bridges. Target of beta-lactam antibiotics.
Bacterial Cell Wall
The unique bacterial structure that antibiotics exploit
N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG) form the glycan backbone. Peptide cross-links added by transpeptidase (penicillin-binding protein). Beta-lactams (penicillin, amoxicillin) block transpeptidase → weak wall → cell lysis. Humans lack peptidoglycan → no toxicity to us.
Three ways bacteria share DNA — including antibiotic resistance genes
Transformation: uptake of naked DNA from environment (Streptococcus pneumoniae). Transduction: bacteriophage carries DNA between cells. Conjugation: direct cell-to-cell transfer via pilus — most important for spreading resistance plasmids. R plasmids carry multiple resistance genes.
The antiphagocytic shield of encapsulated bacteria
Capsule surrounds the cell wall — made of polysaccharide (or protein in B. anthracis). Prevents opsonization and phagocytosis by macrophages. Key encapsulated pathogens: S. pneumoniae, H. influenzae, N. meningitidis, Klebsiella — "SHiNE SKiS". Capsule vaccines (Prevnar, Hib) induce anticapsular antibodies.
Obligate vs Facultative
Obligate aerobe needs O₂. Obligate anaerobe dies with O₂. Facultative anaerobe: can do both.
Oxygen Requirements
Bacterial oxygen tolerance determines where infections occur
Obligate aerobes: Mycobacterium tuberculosis (upper lung lobes — high O₂). Obligate anaerobes: Clostridium, Bacteroides — found in deep wounds, gut, abscesses. Facultative anaerobes: E. coli, Staph — grow anywhere. Microaerophiles (Campylobacter): low O₂ preferred.
Two classes of bacterial toxins — very different mechanisms
Exotoxins: proteins secreted by living bacteria. Heat-labile, highly specific. Examples: botulinum toxin, cholera toxin (cAMP ↑), diphtheria toxin (EF-2 inhibition), tetanospasmin. Endotoxins: LPS from Gram- cell walls. Released on cell death. Heat-stable. Causes fever, shock, DIC.
Biofilm
Biofilm = bacterial communities in polysaccharide matrix — resistant to antibiotics and immune attack
Bacterial Biofilm
Why chronic infections on implants and catheters are so hard to treat
Biofilm: bacteria attach to surface, produce extracellular polysaccharide matrix. 10–1000× more resistant to antibiotics than planktonic (free-floating) bacteria. Common on prosthetic valves, urinary catheters, dental plaque. S. epidermidis and S. aureus are major biofilm formers. Often requires device removal.
Minimum Inhibitory Concentration
MIC = lowest antibiotic concentration that stops visible growth. MBC = kills 99.9% of bacteria.
MIC / MBC
Lab values that guide antibiotic dosing decisions
MIC (minimum inhibitory concentration): bacteria stop growing but may survive. MBC (minimum bactericidal concentration): kills ≥99.9% of original inoculum. Bacteriostatic drugs (tetracycline, chloramphenicol): MBC >> MIC. Bactericidal drugs (penicillin, aminoglycosides): MBC ≈ MIC. Disk diffusion (Kirby-Bauer) is the clinical shortcut.