Study Guide: Pathogenic Gram-Positive and Gram-Negative Bacteria

Gram-Positive Bacteria

Staphylococcus

Staphylococcus species are Gram-positive, spherical bacteria that form clusters resembling grapes. They are normal members of the human microbiota but can become opportunistic pathogens, causing a range of diseases from minor skin infections to life-threatening systemic illnesses.

• Structure and Physiology: Gram-positive, anaerobic, salt-tolerant, and resistant to radiation, heat, and desiccation.

• Pathogenicity: Depends on evasion of phagocytosis, production of enzymes, and toxins.

• Structural Defenses: Protein A inhibits opsonization and complement cascade; coagulase forms blood clots to hide bacteria; capsules inhibit chemotaxis.

• Enzymes: Coagulase (S. aureus only), hyaluronidase, staphylokinase, lipase, penicillinase.

• Toxins: Cytolytic toxins (alpha, beta, gamma, delta, leukocidin), exfoliative toxins, toxic shock syndrome toxin, enterotoxins (heat stable).

• Epidemiology: S. epidermidis is ubiquitous on skin; S. aureus is more common in skin folds. Transmission via direct contact and fomites.

• Diseases: Food poisoning, cutaneous diseases (scalded skin syndrome, impetigo, folliculitis), systemic diseases (toxic shock syndrome, bacteremia, endocarditis, pneumonia, osteomyelitis).

• Diagnosis: Coagulase test differentiates S. aureus (positive) from S. epidermidis (negative).

• Treatment: Vancomycin for MRSA; removal of pus and bacteria; long-term therapy for systemic infections.

• Prevention: Frequent handwashing; staph cannot be eliminated due to its presence on skin.

Streptococcus

Streptococcus species are Gram-positive cocci arranged in pairs or chains. They are classified by Lancefield antigens, hemolysis type, and other properties.

• Group A (S. pyogenes): Possess M proteins, hyaluronic acid capsule, C5A peptidase, hyaluronidase, and secrete pyrogenic toxins and streptolysins.

• Epidemiology: Infects larynx and skin; spread via respiratory droplets; sensitive to penicillins.

• Diseases: Pharyngitis, scarlet fever, rheumatic fever, pyoderma, erysipelas, cellulitis, toxic shock syndrome, necrotizing fasciitis, glomerulonephritis.

• Diagnosis: Rapid strep tests; penicillin effective; surgical removal for necrotizing fasciitis.

• Group B (S. agalactiae): Pathogenic in newborns; causes neonatal bacteremia, meningitis, pneumonia.

• Other Beta Hemolytic: S. equisimilis and S. anginosus cause pharyngitis.

• Alpha Hemolytic (Viridians Group): Cause dental caries, meningitis, endocarditis.

• S. pneumoniae: Causes pneumonia, sinusitis, otitis media, bacteremia, endocarditis, meningitis; diagnosed by Quellung reaction and bile solubility.

• Enterococcus: Lancefield group D; cause HAIs, bacteremia, endocarditis, wound infections; resistant to many antibiotics.

Bacillus

Bacillus species are rod-shaped, endospore-forming bacteria found in soil. Pathogenic strains cause anthrax via toxins that disrupt cellular metabolism.

• Disease: Anthrax (cutaneous, GI, inhalation).

• Treatment: Antimicrobial drugs do not neutralize accumulated toxin.

Clostridium

Clostridium are anaerobic, endospore-forming bacilli. They produce potent exotoxins and cause diseases such as pseudomembranous colitis.

• Toxins: Toxin A (diarrhea), Toxin B (cytotoxic).

• Treatment: Discontinue causative antibiotics; metronidazole or vancomycin.

Listeria

Listeria is tolerant of cold and can grow on refrigerated foods. Causes listeriosis, especially in immunocompromised patients.

Corynebacterium

High G+C, non-endospore forming bacteria. Causes diphtheria via toxin that inhibits protein synthesis.

• Treatment: Antitoxin, penicillin, erythromycin; immunization (DTaP vaccine).

Propionibacterium

Small anaerobic rods; cause acne by growing in oil glands. Treatment includes clindamycin, erythromycin, retinoids.

Nocardia and Actinomyces

Filamentous bacteria resembling fungal hyphae. Nocardia is difficult to stain; Actinomyces stains purple. Treatment involves prolonged antibiotics and surgical removal.

Gram-Negative Bacteria

Intro to Gram-Negative Bacteria

Gram-negative bacteria are diverse and include many human pathogens. Their cell wall contains Lipid A, a component of endotoxin responsible for fever, vasodilation, shock, and disseminated intravascular coagulation (DIC).

• Classification: Grouped by DNA similarities, shape, oxygen requirements, and biochemical characteristics.

• Clinical Organization: Gram-negative cocci, facultatively anaerobic bacilli, aerobic bacilli, strictly anaerobic bacilli.

Neisseria

The only genus of Gram-negative cocci that commonly causes human disease. Includes N. gonorrhoeae (gonorrhea) and N. meningitidis (meningitis).

• Structure: Gram-negative diplococci, coffee bean shape, nonmotile, aerobic, oxidase positive.

• Virulence: Fimbriae, polysaccharide capsule, lipooligosaccharide (LOS) with Lipid A.

• Lab Characteristics: Fastidious; require chocolate or Thayer Martin agar; sensitive to drying and temperature.

• N. gonorrhoeae: Causes gonorrhea, pelvic inflammatory disease, neonatal eye infection (ophthalmia neonatorum).

• Diagnosis: Microscopy, genetic probe, culture.

• Treatment: Ceftriaxone plus azithromycin or doxycycline; resistance is common.

• N. meningitidis: Causes meningitis and septicemia; diagnosed by spinal tap and maltose fermentation.

• Treatment: Immediate cephalosporin; prevention via antibiotics and vaccination.

Facultatively Anaerobic Bacilli

Includes Enterobacteriaceae (oxidase negative) and Pasteurellaceae (oxidase positive). Major pathogens cause HAIs and are distinguished by biochemical tests.

• Enterobacteriaceae: LPS with Lipid A, capsules, fimbriae, adhesins, exotoxins, siderophores, hemolysins, type III secretion system, drug resistance.

• Diagnosis: Selective and differential media (EMB, MacConkey).

• Treatment: Hydration for diarrhea; antibiotics for internal infections; resistance is common.

• Prevention: Hygiene, sewage control, prophylaxis.

Coliform Opportunists

Gram-negative rods that ferment lactose; normal microbiota and opportunists. Includes Escherichia, Klebsiella, Serratia, Enterobacter, Hafnia, Citrobacter.

• E. coli: Causes gastroenteritis, UTIs, septicemia, neonatal meningitis, pneumonia. EHEC produces shiga-like toxin.

• Klebsiella: Nonmotile, large capsule, causes pneumonia, UTIs, meningitis.

• Serratia: Motile, red pigment, opportunistic HAIs.

NonColiform Opportunists

Do not ferment lactose; include Proteus, Morganella, Providencia, Edwardsiella. Major cause of UTIs and infection stones.

True Pathogens

Almost always pathogenic; include Salmonella, Shigella, Yersinia. Use type III secretion systems to disrupt host defenses.

• Salmonella: Causes salmonellosis and typhoid fever.

• Shigella: Causes shigellosis; S. dysenteriae produces shiga toxin.

• Yersinia: Causes enteric illness and plague (bubonic, pneumonic).

Pasteurellaceae

Oxidase positive, small, nonmotile, require heme/cytochromes. Includes Pasteurella and Haemophilus.

• Pasteurella: Infection via animal bites; causes local inflammation and bacteremia.

• Haemophilus influenzae type b (Hib): Causes meningitis, cellulitis, arthritis, epiglottitis; Hib vaccine has reduced disease.

Pathogenic Gram-Negative Anaerobic Bacilli

Dominant microbiota in GI, urinary, reproductive, and lower respiratory tracts. Opportunistic when moved to new sites via trauma or surgery.

• Bacteroides: Bile tolerant; main species is B. fragilis; causes GI disease, wound infections, bacteremia.

• Prevotella: Bile sensitive; mainly in urinary, genital, and respiratory tracts; causes ear, periodontal, gynecological, brain, and abdominal infections.

Summary Table: Gram-Negative Pathogenic Bacteria

Group	Main Genera	Key Features	Diseases
Gram-Negative Cocci	Neisseria	Fimbriae, capsule, LOS	Gonorrhea, meningitis
Facultative Anaerobic Bacilli	Enterobacteriaceae, Pasteurellaceae	LPS, capsules, exotoxins	UTIs, gastroenteritis, pneumonia, meningitis
Aerobic Bacilli	Pseudomonas, Bartonella, Brucella, Bordetella	Various toxins	HAIs, trench fever, brucellosis, whooping cough
Anaerobic Bacilli	Bacteroides, Prevotella	Bile tolerance, capsule	GI, wound, gynecological infections

Additional info: The included image illustrates the effects of Lipid A endotoxin, showing its role in triggering fever, inflammation, DIC, and shock via cytokine release, complement activation, and blood-clotting reactions.