Study Notes: Pathogenic Bacteria – Gram-Positive, Gram-Negative, and Other Notable Groups

Gram-Positive Pathogenic Bacteria

Overview of Gram-Positive Pathogens

Gram-positive bacteria possess a thick peptidoglycan cell wall and retain the crystal violet stain in the Gram staining procedure. Several genera include important human pathogens responsible for a variety of diseases. Understanding their distinguishing features, virulence factors, and associated diseases is essential for microbiology students.

• Staphylococcus: S. aureus, S. epidermidis

• Streptococcus: S. pyogenes, S. agalactiae, S. pneumoniae

• Enterococcus: E. faecalis

• Bacillus: B. anthracis

• Clostridium: C. difficile, C. botulinum, C. tetani, C. perfringens

• Listeria: L. monocytogenes

• Mycoplasma: M. pneumoniae

• Corynebacterium: C. diphtheriae

• Mycobacterium: M. tuberculosis, M. leprae

Virulence Factors of Staphylococcus aureus

• Structural Defenses: Protein A (binds Fc region of antibodies), capsule (inhibits phagocytosis)

• Enzymes: Coagulase (clots plasma), hyaluronidase (spreads infection), staphylokinase (dissolves clots), lipases, beta-lactamase

• Toxins: Cytolytic toxins, exfoliative toxins, toxic shock syndrome toxin, enterotoxins

• Diseases: Skin infections (impetigo, boils), food poisoning, toxic shock syndrome, scalded skin syndrome

Staphylococci vs. Streptococci

• Staphylococci: Clusters, catalase-positive, common on skin and mucous membranes

• Streptococci: Chains or pairs, catalase-negative, often found in the throat and respiratory tract

• Diseases: Staphylococci cause abscesses, food poisoning; Streptococci cause pharyngitis, scarlet fever, pneumonia, neonatal sepsis

Lancefield Classification of Streptococci

The Lancefield system classifies streptococci based on cell wall carbohydrate antigens:

• Group A: S. pyogenes – causes strep throat, rheumatic fever, scarlet fever

• Group B: S. agalactiae – causes neonatal sepsis and meningitis

• Non-groupable: S. pneumoniae – causes pneumonia, otitis media, meningitis

Transmission, Treatment, and Prevention (General Principles)

• Transmission: Direct contact, respiratory droplets, contaminated food/water, wounds

• Treatment: Antibiotics (specifics vary), supportive care

• Prevention: Hygiene, vaccination (where available), food safety, wound care

Unique Characteristics and Diseases

Bacterium	Unique Feature	Disease(s)
Bacillus anthracis	Spore-forming, capsule, anthrax toxin	Anthrax (cutaneous, inhalational, gastrointestinal)
Clostridium botulinum	Produces botulinum neurotoxin	Botulism (flaccid paralysis)
Clostridium tetani	Tetanospasmin toxin	Tetanus (spastic paralysis)
Clostridium difficile	Toxin A and B, pseudomembranous colitis	Antibiotic-associated diarrhea
Listeria monocytogenes	Intracellular, grows at low temperatures	Listeriosis (meningitis, sepsis in neonates/pregnant)
Mycoplasma pneumoniae	Lacks cell wall	"Walking" pneumonia
Corynebacterium diphtheriae	Diphtheria toxin, pseudomembrane formation	Diphtheria
Mycobacterium tuberculosis	Acid-fast, mycolic acids in cell wall	Tuberculosis
Mycobacterium leprae	Obligate intracellular, slow-growing	Leprosy (Hansen's disease)

Gram-Negative Pathogenic Bacteria

Overview of Gram-Negative Pathogens

Gram-negative bacteria have a thin peptidoglycan layer and an outer membrane containing lipopolysaccharide (LPS). They include both cocci and bacilli, many of which are significant human pathogens with diverse virulence mechanisms.

• Neisseria: N. gonorrhoeae, N. meningitidis

• Enterobacteriaceae: E. coli, K. pneumoniae, S. marcescens, Enterobacter, P. mirabilis, Salmonella, Shigella, Yersinia

• Other notable genera: Haemophilus, Bartonella, Brucella, Bordetella, Pseudomonas, Francisella, Legionella, Coxiella

Cocci vs. Bacilli and Unique Features

Bacterium	Shape	Unique Feature	Disease(s)
Neisseria gonorrhoeae	Coccus	Diplococcus, pili, antigenic variation	Gonorrhea
Neisseria meningitidis	Coccus	Capsule, endotoxin	Meningitis, meningococcemia
Escherichia coli	Bacillus	Lactose fermenter, O/H/K antigens	UTIs, gastroenteritis, neonatal meningitis
Klebsiella pneumoniae	Bacillus	Large capsule, mucoid colonies	Pneumonia, UTIs
Serratia marcescens	Bacillus	Red pigment (prodigiosin)	Nosocomial infections
Pseudomonas aeruginosa	Bacillus	Blue-green pigment (pyocyanin), fruity odor	Burn/wound infections, pneumonia
Bordetella pertussis	Bacillus	Pertussis toxin	Whooping cough
Yersinia pestis	Bacillus	Bipolar staining, flea vector	Plague
Legionella pneumophila	Bacillus	Intracellular, water systems	Legionnaires' disease

Transmission, Treatment, and Prevention (General Principles)

• Transmission: Respiratory droplets, contaminated water/food, vectors (fleas, ticks), direct contact

• Treatment: Antibiotics (varies by organism), supportive care

• Prevention: Vaccination (where available), sanitation, vector control, safe food/water practices

Unique Characteristics and Diseases

• Bordetella pertussis: Causes pertussis (whooping cough) via pertussis toxin

• Pseudomonas aeruginosa: Produces pyocyanin (blue-green pigment), associated with burn and respiratory infections

• Yersinia pestis: Causes plague, transmitted by fleas

• Legionella pneumophila: Causes Legionnaires' disease, associated with water systems

Rickettsias, Spirochetes, Chlamydias, and Vibrios

Overview of Notable Non-Enteric Pathogens

This group includes obligate intracellular bacteria, spirochetes, and curved rods, many of which cause systemic or gastrointestinal diseases. Their unique biology often influences transmission and treatment strategies.

• Rickettsia rickettsii: Rocky Mountain spotted fever

• Chlamydia trachomatis: Trachoma, chlamydia (STD)

• Treponema pallidum: Syphilis

• Borrelia burgdorferi: Lyme disease

• Leptospira interrogans: Leptospirosis

• Vibrio cholerae: Cholera

• Campylobacter jejuni: Gastroenteritis

• Helicobacter pylori: Peptic ulcers, gastritis

Transmission, Treatment, and Prevention (General Principles)

• Transmission: Arthropod vectors (ticks, lice), sexual contact, contaminated water/food, animal urine

• Treatment: Antibiotics (specifics vary), supportive care

• Prevention: Vector control, safe sex practices, sanitation, avoiding contaminated water

Unique Characteristics and Diseases

Organism	Unique Feature	Disease(s)
Rickettsia rickettsii	Obligate intracellular, tick vector	Rocky Mountain spotted fever
Chlamydia trachomatis	Obligate intracellular, lacks peptidoglycan	Chlamydia, trachoma
Treponema pallidum	Spirochete, cannot be cultured in vitro	Syphilis
Borrelia burgdorferi	Spirochete, tick vector	Lyme disease
Leptospira interrogans	Hooked ends, animal urine transmission	Leptospirosis
Vibrio cholerae	Comma-shaped, cholera toxin	Cholera (severe watery diarrhea)
Campylobacter jejuni	Curved rod, microaerophilic	Bacterial gastroenteritis
Helicobacter pylori	Urease production, colonizes stomach	Peptic ulcers, gastritis

Example: Helicobacter pylori and Peptic Ulcers

• Unique Feature: Produces urease to neutralize stomach acid

• Disease: Peptic ulcers, gastritis

• Transmission: Fecal-oral route

• Prevention: Improved sanitation, safe food and water

Additional info: For all organisms, specific antibiotic regimens are not required at this level, but understanding general mechanisms of transmission, prevention, and unique pathogenic features is essential for exam preparation.