Chapter 19: Pathogenic Gram-Positive Bacteria

Staphylococci: Basic Characteristics

• Staphylococci are Gram-positive cocci that form clusters resembling grapes.

• They are facultative anaerobes and catalase-positive.

• Commonly found on the skin and mucous membranes.

• Notable species: Staphylococcus aureus (pathogenic), S. epidermidis (less pathogenic).

Diseases Caused by Staphylococcus aureus

• Skin infections: boils, impetigo, cellulitis.

• Systemic infections: bacteremia, endocarditis, osteomyelitis.

• Toxin-mediated diseases: toxic shock syndrome, scalded skin syndrome, food poisoning.

• Virulence factors: coagulase, protein A, hemolysins, enterotoxins.

Streptococcus: Basic Characteristics

• Gram-positive cocci arranged in chains or pairs.

• Catalase-negative, facultative anaerobes.

• Classified by hemolysis on blood agar: alpha (partial), beta (complete), gamma (none).

• Important species: Streptococcus pyogenes (Group A), S. mutans, S. pneumoniae.

Diseases Caused by Streptococcus pyogenes

• Pharyngitis (strep throat), scarlet fever, impetigo, erysipelas, necrotizing fasciitis.

• Post-infection sequelae: rheumatic fever (autoimmune), glomerulonephritis.

• Rheumatic fever results from immune cross-reactivity after untreated strep throat.

Streptococcus mutans and Biofilm Formation

• S. mutans initiates dental biofilm (plaque) by adhering to tooth surfaces and producing extracellular polysaccharides from sucrose.

• Fermentation of sugars produces acid, leading to tooth decay (dental caries).

Anthrax

• Caused by Bacillus anthracis, a spore-forming Gram-positive rod.

• Three types: cutaneous, inhalational (pulmonary), gastrointestinal.

• Found in soil; spores can infect humans and animals.

Bt-toxin

• Produced by Bacillus thuringiensis.

• Used as a biological insecticide; toxic to certain insect larvae but not to humans.

Clostridium Species

• Clostridium tetani: Causes tetanus; produces tetanospasmin toxin causing muscle spasms.

• C. botulinum: Causes botulism; produces botulinum toxin, leading to flaccid paralysis.

• Both are anaerobic, spore-forming rods found in soil.

Listeria

• Listeria monocytogenes is a Gram-positive rod.

• Can grow at refrigeration temperatures; transmitted via contaminated food (e.g., raw milk, soft cheeses).

• Causes listeriosis, especially dangerous for pregnant women, newborns, and immunocompromised individuals.

Mycobacterium

• Acid-fast rods with waxy cell walls (mycolic acids).

• Mycobacterium tuberculosis: causes tuberculosis; diagnosed by tuberculin skin test, chest X-ray, acid-fast stain.

• M. leprae: causes leprosy (Hansen's disease).

• Pathogenesis involves survival within macrophages.

Acne

• Caused by Cutibacterium acnes (formerly Propionibacterium acnes).

• Pathogenesis: colonization of hair follicles, inflammation due to bacterial metabolism of sebum.

Mycoplasmas

• Lack a cell wall; smallest free-living bacteria.

• Pleomorphic; resistant to antibiotics targeting cell wall synthesis (e.g., penicillins).

• Cause atypical pneumonia (Mycoplasma pneumoniae).

Chapter 20: Pathogenic Gram-Negative Bacteria

Endotoxin vs. Exotoxin

• Endotoxin: Lipid A component of lipopolysaccharide (LPS) in outer membrane of Gram-negative bacteria; released upon cell lysis.

• Exotoxin: Proteins secreted by bacteria (both Gram-positive and Gram-negative); highly specific effects.

Effects of Lipid A

• Triggers strong immune response: fever, inflammation, shock (endotoxic shock), disseminated intravascular coagulation (DIC).

Neisseria gonorrhoeae

• Gram-negative diplococcus.

• Infects mucous membranes of the reproductive tract.

• In males: urethritis, purulent discharge.

• In females: often asymptomatic, can cause pelvic inflammatory disease (PID), infertility.

• Can cause neonatal conjunctivitis.

Neisseria meningitidis

• Spread by respiratory droplets; colonizes nasopharynx.

• Symptoms: sudden fever, headache, stiff neck, petechial rash.

• Diagnosed by Gram stain and culture of cerebrospinal fluid.

• Virulence factors: capsule, endotoxin, IgA protease.

Enteric Bacteria

• Virulence factors: fimbriae, toxins, capsules, antigenic variation.

• Prevention: proper food handling, sanitation, handwashing.

• Escherichia coli: causes diarrhea, urinary tract infections, neonatal meningitis, hemolytic uremic syndrome (HUS).

Salmonellosis and Typhoid Fever

• Caused by Salmonella enterica (various serotypes).

• Salmonellosis: gastroenteritis from contaminated food (poultry, eggs).

• Typhoid fever: systemic infection by S. Typhi; transmitted via fecal-oral route.

Yersinia pestis (Plague)

• Endemic in wild rodents; transmitted by flea bites.

• Bubonic plague: swollen lymph nodes (buboes), fever.

• Pneumonic plague: lung infection, highly contagious, fatal if untreated.

• Virulence: antiphagocytic proteins, endotoxin.

Pasteurella multocida and P. haemolytica

• Infect animals (cattle, sheep, pets); can be transmitted to humans via bites or scratches.

• Reservoirs for zoonotic infections.

Bordetella pertussis (Whooping Cough)

• Causes pertussis (whooping cough).

• Symptoms: severe coughing fits, inspiratory "whoop" sound.

• Virulence: pertussis toxin, tracheal cytotoxin.

Legionella pneumophila

• Survives in aquatic environments, often in biofilms or within amoebae.

• Transmitted via inhalation of contaminated water aerosols (e.g., air conditioning systems).

• Causes Legionnaires' disease (pneumonia).

Bacteroides

• Obligate anaerobes; Gram-negative rods.

• Normal flora of the gut; can cause opportunistic infections if displaced (e.g., after surgery).

Chapter 21: Pathogenic Intracellular Bacteria and Others

Rickettsia

• Obligate intracellular Gram-negative rods.

• Transmitted by arthropod vectors (ticks, lice, fleas).

• Diseases: Rocky Mountain spotted fever (RMSF), typhus.

Chlamydia

• Obligate intracellular bacteria; unique developmental cycle with two forms:

  • Elementary body (EB): infectious, extracellular form.

  • Reticulate body (RB): replicative, intracellular form.

• Most common form: EB (infectious).

• STD (Lymphogranuloma venereum) caused by Chlamydia trachomatis (L1-L3 serovars).

• Pneumonia caused by C. pneumoniae or C. psittaci.

• Transmitted via sexual contact, respiratory droplets, or fomites.

Spirochetes

• Gram-negative, spiral-shaped bacteria with axial filaments for motility.

• Major diseases: syphilis (Treponema pallidum), Lyme disease (Borrelia burgdorferi), leptospirosis (Leptospira interrogans).

Vibrios

• Comma-shaped, Gram-negative rods; motile with a single polar flagellum.

• Example: Vibrio cholerae (causes cholera).

• Cholera toxin: AB toxin that increases cAMP in intestinal cells, causing watery diarrhea.

• CrvA: Protein involved in cell curvature (Additional info: CrvA is a cytoskeletal protein in vibrios).

Campylobacter jejuni and Helicobacter pylori

• Campylobacter jejuni: curved, Gram-negative rod; causes gastroenteritis, often from undercooked poultry.

• Helicobacter pylori: spiral-shaped, Gram-negative; colonizes stomach, causes gastritis, peptic ulcers, and is associated with gastric cancer.

Table: Comparison of Endotoxins and Exotoxins