Gram-positive cocci and rods cause many familiar diseases. Streptococcus pyogenes (Group A Streptococcus) is the leading cause of bacterial pharyngitis and can lead to post-streptococcal complications including rheumatic fever and glomerulonephritis; it also produces erythrogenic (pyrogenic) exotoxin responsible for scarlet fever. Streptococcus pneumoniae, alpha-hemolytic, lancet-shaped, and optochin-sensitive, is a major cause of bacterial meningitis and pneumonia. Streptococcus agalactiae (Group B Streptococcus) causes neonatal meningitis and sepsis. Staphylococcus aureus, coagulase-positive and catalase-positive, causes skin infections, food poisoning, and toxic shock syndrome via the superantigen TSST-1. The coagulase-negative Staphylococcus epidermidis commonly infects prosthetic devices and catheters. Bacillus anthracis causes anthrax through three toxin components, protective antigen, edema factor, and lethal factor. Clostridium tetani produces tetanospasmin, which blocks inhibitory neurotransmitters and causes spastic paralysis. Clostridium botulinum releases botulinum toxin that blocks acetylcholine release at the neuromuscular junction, causing flaccid paralysis. Clostridium perfringens causes gas gangrene via alpha toxin, a lecithinase. Clostridioides difficile causes pseudomembranous colitis, often after antibiotic use disrupts normal flora. Corynebacterium diphtheriae produces diphtheria toxin, which ADP-ribosylates elongation factor-2 (EF-2), inhibiting host protein synthesis. The acid-fast Mycobacterium tuberculosis causes tuberculosis, with primary infection sometimes producing a calcified Ghon complex. Mycobacterium leprae causes Hansen's disease (leprosy). Listeria monocytogenes causes listeriosis with characteristic growth at refrigeration temperatures and transplacental transmission, leading to neonatal meningitis.
Gram-negative pathogens span cocci, bacilli, and curved forms. Neisseria gonorrhoeae causes gonorrhea, and Neisseria meningitidis causes bacterial meningitis with a characteristic petechial rash. Escherichia coli is the most common cause of community-acquired urinary tract infections, and enterohemorrhagic E. coli (EHEC), especially O157:H7, produces Shiga-like toxin causing hemolytic uremic syndrome, while enterotoxigenic E. coli (ETEC) causes traveler's diarrhea. Salmonella typhi causes typhoid fever, while non-typhoidal Salmonella such as S. enteritidis causes gastroenteritis from contaminated poultry and eggs. Shigella species cause bacillary dysentery. Vibrio cholerae produces cholera toxin that activates adenylate cyclase, raising cAMP and causing massive watery diarrhea. Campylobacter jejuni, acquired from undercooked poultry, is a leading cause of bacterial gastroenteritis. Helicobacter pylori causes peptic ulcers and gastritis. Pseudomonas aeruginosa, a major opportunistic pathogen in cystic fibrosis and burn patients, produces a characteristic blue-green pigment. Bordetella pertussis causes whooping cough. Legionella pneumophila causes Legionnaires' disease, often from contaminated water systems such as cooling towers. Klebsiella pneumoniae causes community-acquired pneumonia with mucoid colonies. Haemophilus influenzae type b (Hib) causes epiglottitis and meningitis, especially in unvaccinated children. Yersinia pestis causes plague, transmitted by fleas.
Spirochetes and other distinctive pathogens include Treponema pallidum, which causes syphilis through stages of chancre, rash and systemic symptoms, latent disease, and eventually tertiary gummas with cardiovascular and neurological involvement. Borrelia burgdorferi causes Lyme disease, transmitted by Ixodes ticks, with erythema migrans (bullseye rash) as the classic early finding. Leptospira interrogans causes leptospirosis, often from water contaminated with animal urine. Chlamydia trachomatis is an obligate intracellular bacterium causing trachoma and the most common bacterial STI worldwide. Chlamydophila pneumoniae causes atypical pneumonia, as does the cell-wall-deficient Mycoplasma pneumoniae. Among Rickettsia, obligate intracellular bacteria that cannot be cultured on standard media, Rickettsia rickettsii causes Rocky Mountain spotted fever. Coxiella burnetii causes Q fever and uniquely survives outside host cells via a spore-like form. Other notable pathogens include Francisella tularensis (tularemia), Brucella species (brucellosis from unpasteurized dairy), Bartonella henselae (cat scratch disease), and Bacteroides fragilis, an anaerobic rod that causes abscesses when it breaches normal oral or gastrointestinal flora. Bacterial toxins fall into two broad categories: exotoxins, secreted proteins with specific molecular targets and high potency, often produced by Gram-positive organisms, and endotoxins, which are lipopolysaccharide (LPS) released upon Gram-negative cell lysis, causing fever and septic shock. Superantigens such as TSST-1 and streptococcal pyrogenic exotoxin bridge MHC II and T-cell receptors outside the normal antigen-binding groove, triggering massive non-specific T-cell activation and cytokine release.