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 wide range of diseases.

• 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 Features: Protein A (binds Fc region of IgG), capsule (inhibits phagocytosis)

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

• Toxins: Hemolysins, leukocidins, toxic shock syndrome toxin (TSST-1), enterotoxins (cause food poisoning), exfoliative toxins (cause scalded skin syndrome)

Staphylococci vs. Streptococci

• Staphylococci: Grape-like clusters, catalase positive, common on skin and mucous membranes

• Streptococci: Chains or pairs, catalase negative, classified by hemolysis (alpha, beta, gamma) and Lancefield groups

• Diseases: S. aureus causes skin infections, pneumonia, endocarditis, food poisoning; S. pyogenes causes pharyngitis, scarlet fever, rheumatic fever, necrotizing fasciitis

Lancefield Classification of Streptococci

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

• Group A: S. pyogenes – causes pharyngitis, impetigo, rheumatic fever

• Group B: S. agalactiae – neonatal sepsis, meningitis

• Other: S. pneumoniae (no Lancefield antigen) – pneumonia, otitis media, meningitis

Transmission, Treatment, and Prevention (General Principles)

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

• Treatment: Antibiotics (specifics vary), supportive care

• Prevention: Hygiene, vaccination (where available), food safety, isolation of infected individuals

Unique Characteristics of Selected Gram-Positive Pathogens

• Bacillus anthracis: Causes anthrax; forms spores; transmitted via inhalation, ingestion, or cutaneous exposure

• Clostridium botulinum: Produces botulinum toxin; causes botulism (flaccid paralysis); associated with improperly canned foods

• Clostridium tetani: Produces tetanospasmin; causes tetanus (spastic paralysis); enters via wounds

• Clostridium difficile: Causes antibiotic-associated colitis; produces toxins A and B

• Listeria monocytogenes: Causes listeriosis; can cross placenta; associated with contaminated dairy and deli meats

• Mycoplasma pneumoniae: Lacks cell wall; causes atypical (walking) pneumonia

• Corynebacterium diphtheriae: Produces diphtheria toxin; causes diphtheria (pseudomembrane in throat)

• Mycobacterium tuberculosis: Causes tuberculosis; acid-fast bacillus; transmitted via aerosols

• Mycobacterium leprae: Causes leprosy (Hansen's disease); affects skin and nerves

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.

• Neisseria: N. gonorrhoeae, N. meningitidis (cocci)

• Enterobacteriaceae: Escherichia coli, Klebsiella pneumoniae, Serratia marcescens, Enterobacter, Proteus mirabilis, Salmonella enterica (serovars Typhi, Typhimurium), Shigella sonnei, Yersinia pestis

• Other Bacilli: Haemophilus influenzae, Bartonella henselae, Brucella melitensis, Bordetella pertussis, Pseudomonas aeruginosa, Francisella tularensis, Legionella pneumophila, Coxiella burnetii

Cocci vs. Bacilli and Unique Features

Bacterium	Shape	Unique Feature
Neisseria	Cocci	Diplococci; N. gonorrhoeae causes gonorrhea; N. meningitidis causes meningitis
Pseudomonas aeruginosa	Bacillus	Produces blue-green pigment (pyocyanin); opportunistic pathogen
Klebsiella pneumoniae	Bacillus	Prominent capsule; causes pneumonia with "currant jelly" sputum
Yersinia pestis	Bacillus	Causes plague; transmitted by fleas
Bordetella pertussis	Bacillus	Causes whooping cough; produces pertussis toxin
Legionella pneumophila	Bacillus	Causes Legionnaires' disease; found in water systems
Coxiella burnetii	Bacillus	Causes Q fever; transmitted by aerosols from livestock
Haemophilus influenzae	Bacillus	Causes meningitis, epiglottitis; requires X and V factors for growth

Transmission, Treatment, and Prevention (General Principles)

• Transmission: Respiratory droplets, contaminated food/water, vectors (e.g., fleas for Y. pestis), direct contact

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

• Prevention: Vaccination (where available), sanitation, vector control, safe food handling

Unique Characteristics of Selected Gram-Negative Pathogens

• Escherichia coli: Causes urinary tract infections, gastroenteritis; some strains produce Shiga toxin

• Salmonella enterica: Serovar Typhi causes typhoid fever; serovar Typhimurium causes gastroenteritis

• Shigella sonnei: Causes bacillary dysentery (shigellosis); low infectious dose

• Bartonella henselae: Causes cat scratch disease

• Brucella melitensis: Causes brucellosis (undulant fever); transmitted via unpasteurized dairy

• Francisella tularensis: Causes tularemia; transmitted by ticks, rabbits

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.

• Rickettsia rickettsii: Causes Rocky Mountain spotted fever

• Chlamydia trachomatis: Causes chlamydia (STD), trachoma (eye infection)

• Treponema pallidum: Causes syphilis

• Borrelia burgdorferi: Causes Lyme disease

• Leptospira interrogans: Causes leptospirosis

• Vibrio cholerae: Causes cholera (severe watery diarrhea)

• Campylobacter jejuni: Causes gastroenteritis

• Helicobacter pylori: Causes peptic ulcers, gastritis

Transmission, Treatment, and Prevention (General Principles)

• Transmission: Arthropod vectors (e.g., ticks for R. rickettsii, B. burgdorferi), sexual contact (C. trachomatis, T. pallidum), contaminated water (V. cholerae), animal urine (L. interrogans), undercooked poultry (C. jejuni), oral-oral or fecal-oral (H. pylori)

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

• Prevention: Vector control, safe sex practices, water sanitation, food safety, animal control

Unique Characteristics of Selected Pathogens

• Rickettsia rickettsii: Obligate intracellular; causes spotted rash; transmitted by ticks

• Chlamydia trachomatis: Obligate intracellular; causes genital and ocular infections

• Treponema pallidum: Spirochete; causes syphilis; cannot be cultured in vitro

• Borrelia burgdorferi: Spirochete; causes Lyme disease; transmitted by Ixodes ticks

• Leptospira interrogans: Spirochete; causes leptospirosis; transmitted via animal urine

• Vibrio cholerae: Curved rod; produces cholera toxin; causes "rice-water" stools

• Campylobacter jejuni: Curved rod; leading cause of bacterial gastroenteritis

• Helicobacter pylori: Curved rod; produces urease; associated with ulcers and gastric cancer

Summary Table: Selected Pathogens and Diseases

Organism	Disease	Transmission	Unique Feature
Staphylococcus aureus	Skin infections, food poisoning	Direct contact, food	Protein A, coagulase, toxins
Streptococcus pyogenes	Pharyngitis, scarlet fever	Respiratory droplets	Group A, M protein
Bacillus anthracis	Anthrax	Inhalation, cutaneous	Spore-forming
Clostridium botulinum	Botulism	Foodborne	Botulinum toxin
Neisseria meningitidis	Meningitis	Respiratory droplets	Capsule, endotoxin
Escherichia coli	UTI, gastroenteritis	Fecal-oral	Shiga toxin (some strains)
Yersinia pestis	Plague	Flea bite	Buboes, pneumonic form
Bordetella pertussis	Whooping cough	Respiratory droplets	Pertussis toxin
Rickettsia rickettsii	Rocky Mountain spotted fever	Tick bite	Obligate intracellular
Treponema pallidum	Syphilis	Sexual contact	Spirochete
Vibrio cholerae	Cholera	Contaminated water	Cholera toxin
Helicobacter pylori	Peptic ulcers	Oral-oral, fecal-oral	Urease production

Additional info: Academic context and explanations have been expanded for clarity and completeness, including general principles of transmission, treatment, and prevention, and unique features of each pathogen as relevant to undergraduate microbiology.