Medical Microbiology Notes: Gram-Negative Bacilli of Medical Importance

General Characteristics of Aerobic Gram-Negative Nonenteric Bacilli

• Broad Overview:     - This group is comprised of a large and diverse collection of non-spore-forming bacteria.     - Habitats: These bacilli occupy a wide range of environments, including the large intestines of animals (enteric), soil, water, and respiratory tracts. Some are associated with zoonotic transmission.     - Pathogenicity: While most of these organisms are not medically important, the group contains true pathogens as well as opportunists.     - Common Virulence Factor: All members of this group possess an outer membrane lipopolysaccharide (LPS) in their cell wall, which acts as an endotoxin.
• Specific Pathogen Categories:     - Opportunistic Pathogens: Includes Pseudomonas and Burkholderia.     - Zoonotic Pathogens: Includes Brucella and Francisella.     - Mainly Human Pathogens: Includes Bordetella and Legionella.

Classification of Gram-Negative Pathogens

• Gram-Negative Rods Based on Oxygen Requirements and Fermentation:     - Aerobes (Non-fermenters of carbohydrates):         - Pseudomonas         - Bordetella         - Alcaligenes     - Facultative Anaerobes (Fermenters of carbohydrates):         - Lactose Fermenting (Coliforms):             - Escherichia             - Klebsiella             - Citrobacter             - Enterobacter         - Non-Lactose Fermenting (Non-coliforms):             - Oxidase-negative:                 - Salmonella                 - Shigella                 - Proteus                 - Serratia                 - Yersinia             - Oxidase-positive:                 - Haemophilus                 - Pasteurella     - Obligate Anaerobes:         - Bacteroides

Systems Profile of Pathogenic Gram-Negative Bacilli

• Pseudomonas aeruginosa:     - Skin/Skeletal: Complication of burns, skin rashes.     - Nervous/Muscle: Corneal ulcers, meningitis.     - Cardiovascular/Lymphatic/Systemic: Endocarditis.     - Respiratory: Complication of cystic fibrosis.     - Urogenital: Urinary tract infections.
• Bordetella pertussis:     - Respiratory: Whooping cough.
• Legionella spp.:     - Respiratory: Pneumonia and Pontiac fever.
• Escherichia coli:     - Gastrointestinal: Acute diarrhea.     - Urogenital: Urinary tract infections.
• Escherichia coli O157:H7:     - Gastrointestinal: Acute diarrhea and colitis.     - Urogenital: Hemolytic uremic syndrome.
• Klebsiella pneumoniae:     - Skin/Skeletal: Wound infections.     - Nervous/Muscle: Meningitis.     - Respiratory: Pneumonia.     - Urogenital: Urinary tract infections.
• Proteus spp.:     - Skin/Skeletal: Wound infections.     - Respiratory: Pneumonia.     - Urogenital: Urinary tract infections.
• Salmonella typhi:     - Cardiovascular/Lymphatic/Systemic: Septicemia and Typhoid fever.
• Salmonella spp.:     - Gastrointestinal: Acute diarrhea.
• Shigella spp.:     - Nervous/Muscle: Neuron damage.     - Gastrointestinal: Acute diarrhea and bacillary dysentery.
• Yersinia pestis:     - Cardiovascular/Lymphatic/Systemic: Bubonic plague.     - Respiratory: Pneumonic plague.
• Haemophilus influenzae:     - Skin/Skeletal: Conjunctivitis.     - Nervous/Muscle: Meningitis.

Pseudomonas: The Pseudomonads

• General Biology:     - Small gram-negative rods characterized by a single polar flagellum.     - Environment: Free-living organisms found primarily in soil, sea water, and fresh water. They also colonize plants and animals.     - Ecological Role: Important as decomposers and bioremediators.     - Metabolism: Use aerobic respiration; they do not ferment carbohydrates. They are oxidase and catalase positive.     - Pigmentation: Many species produce water-soluble pigments.
• Pseudomonas aeruginosa Characteristics:     - Common inhabitant of soil and water; noted as an intestinal resident in approximately $10\%$ of normal individuals.     - Resilience: Highly resistant to soaps, dyes, quaternary ammonium disinfectants, drugs, and drying.     - Clinical Presence: Frequent contaminant of ventilators, IV solutions, and anesthesia equipment.     - Disease Profile: A leading cause of nosocomial infections, particularly in patients with burns, neoplastic disease, or cystic fibrosis.     - Specific Complications: Pneumonia, UTI, abscesses, otitis, corneal disease, endocarditis, meningitis, and bronchopneumonia.     - Identification markers: Produces a grapelike odor and a greenish-blue pigment known as pyocyanin.     - Treatment Challenge: Noted for being multidrug resistant.

Bordetella pertussis and Whooping Cough

• Morphology: Minute, encapsulated coccobacillus.
• Disease: Causes pertussis (whooping cough), described as an acute respiratory syndrome and a communicable childhood affliction.     - Severity: Often involves life-threatening complications in infants.     - Reservoir: Apparently healthy carriers.     - Transmission: Occurs via direct contact or inhalation of aerosols.
• Virulence Factors:     1. Receptors that recognize and bind specifically to ciliated respiratory epithelial cells.     2. Toxins that destroy and dislodge these ciliated cells.     - Pathophysiology: The loss of the ciliary mechanism results in a buildup of mucus and subsequent blockage of the airways.
• Prevention: Vaccination using DTaP (an acellular vaccine containing toxoid and other specific antigens).
• Epidemiology (U.S. Data 1990–2018):     - Infants (< 1 year): Incidence rates rose from approximately $50$ per $100,000$ in 1990 to $105$ in 2015. The peak occurred in 2012 with approximately $125$ cases per $100,000$.     - Other age groups: Remained below $10$ cases per $100,000$ until 2003; peaked in 2012 with $58$ cases per $100,000$.

Genus Alcaligenes and Legionella pneumophila

• Alcaligenes:     - Live primarily in soil and water and may become part of the normal flora.     - Alcaligenes faecalis: The most common clinical species, isolated from feces, sputum, and urine. It is associated with opportunistic pneumonia, septicemia, and meningitis.
• Legionella pneumophila (Legionellosis):     - Ecology: Widely distributed in water, often in close association with amoebas.     - History: Named after a 1976 outbreak during an American Legion convention that afflicted $200$ members and killed $29$.     - Clinical Manifestations:         - Legionnaires’ disease: The most severe form, resulting in pneumonia with a fatality rate of $3-30\%$.         - Pontiac fever: A milder form of the disease.     - Risk Factors: Prevalent in males over age $50$ and as a nosocomial disease in elderly patients.     - Symptoms: Fever, cough, diarrhea, and abdominal pain. It is notably not contagious (person-to-person).     - Treatment: Levofloxacin or Azithromycin.

Family Enterobacteriaceae: The Enterics

• General Characteristics:     - Large family of small, non-spore-forming, gram-negative rods.     - Distribution: Found in soil, water, decaying matter, and the large intestines of animals.     - Medical Significance: Along with Pseudomonas, they account for nearly $50\%$ of nosocomial infections. They are the most frequent cause of diarrhea via enterotoxins.
• Biochemical Keys:     - Glucose fermentation.     - Nitrate reduction.     - Oxidase-negative.     - Most possess flagella.     - Regular, straight rods.
• Major Subgroups:     - Coliforms: Lactose fermenters (e.g., E. coli).     - Non-coliforms: Non-lactose fermenters.
• Primary Opportunists: Escherichia coli, Klebsiella, Enterobacter, Serratia, Proteus, and Citrobacter.
• The IMViC Tests (Differentiation Table):     - Indole: (+) Escherichia, Citrobacter, Proteus; (-) Serratia, Pseudomonas; (+/-) Klebsiella/Enterobacter.     - Methyl Red: (+) Escherichia, Citrobacter; (-) Klebsiella/Enterobacter, Proteus, Pseudomonas; (Variable) Serratia.     - Voges-Proskauer: (-) Escherichia, Citrobacter, Proteus, Pseudomonas; (+) Serratia; (Variable) Klebsiella/Enterobacter.     - Citrate: (+) Citrobacter, Klebsiella/Enterobacter, Serratia, Proteus; (-) Escherichia; (Variable) Pseudomonas.
• Antigenic Structures:     - H: Flagellar antigen.     - K: Capsule and/or fimbrial antigen (referred to as Vi antigen in Salmonella).     - O: Somatic or cell wall antigen (found in all species; implicated in endotoxic shock).

Escherichia coli: The Most Prevalent Enteric Bacillus

• Statistics: The most common aerobic, non-fastidious bacterium in the gut, with over $150$ identified strains.
• Pathotypes:     - Enterohemorrhagic E. coli (EHEC): Causes hemorrhagic syndrome and kidney damage.     - Enterotoxigenic E. coli (ETEC): Causes severe diarrhea via heat-labile and heat-stable toxins that stimulate fluid secretion; uses fimbriae for attachment.
• Clinical Relevance:     - Frequent agent of infantile diarrhea (major mortality factor for infants).     - Responsible for approximately $70\%$ of traveler's diarrhea.     - Responsible for $50-80\%$ of UTIs.
• Treatment:     - Early oral antimicrobics.     - Over-the-counter preparations (e.g., Imodium) can slow gut motility but may retain the pathogen longer.     - Bismuth salicylate (Pepto-Bismol) is preferred as it counteracts enterotoxins and provides antimicrobial effects.
• E. coli O157:H7:     - Identified in 1982 via fast-food hamburger infections; reservoir is cattle intestines.     - Mechanism: Cell wall receptor fuses with host membrane to secrete toxins. The toxin binds to ribosomes and halts protein synthesis, killing intestinal cells.     - Identification: Rapid testing uses latex beads coated with antibodies specific for E. coli O111 producing Shiga toxin.

Other Coliforms and Noncoliform Lactose-Negative Enterics

• Opportunistic Coliforms:     - Klebsiella pneumoniae: Normal respiratory inhabitant. Features a large capsule. Causes nosocomial pneumonia, meningitis, bacteremia, wound infections, and UTIs.     - Enterobacter sp.: Causes UTIs and surgical wound infections.     - Citrobacter sp.: Causes opportunistic UTIs and bacteremia.     - Serratia marcescens: Produces a characteristic red pigment; causes pneumonia, burn/wound infections, septicemia, and meningitis.
• Noncoliform Lactose-Negative Enterics:     - Proteus sp.: Saprobes in soil/water; commensals in humans. Known for "swarming" on moist agar in concentric patterns. Involved in UTIs, wound infections, and infant diarrhea.     - True Pathogens: Salmonella and Shigella.

True Enteric Pathogens: Salmonella and Shigella

• General Characteristics: Not part of normal human flora; possess well-developed virulence factors.
• Salmonella Taxonomy:     1. Salmonella bongori: Found in cold-blooded animals.     2. Salmonella enterica: Six subspecies (enterica, salamae, arizonae, diarizonae, houtenae, indica).     - There are over $2,500$ serotypes based on source isolates.
• Typhoid Fever (Salmonella serotype Typhi):     - Pathogenesis: Ingested via fecally contaminated food/water. Adheres to the small intestine, causes invasive diarrhea, and enters the blood (septicemia). Gallbladder can harbor chronic bacilli.     - Symptoms: Ulcers and perforations of the intestinal wall.     - Treatment: Ciprofloxacin or ceftriaxone; two vaccines available for temporary protection.
• Non-typhoidal Salmonelloses:     - Incidence peaked in 1986 at $27$ cases per $100,000$ due to contaminated pasteurized milk in Illinois. As of 2018, the rate was approximately $17$ per $100,000$.
• Shigella and Bacillary Dysentery:     - Agents: S. dysenteriae, S. sonnei, S. flexneri, and S. boydii.     - Symptoms: Fever, nausea, abdominal cramps, and watery stool with mucus and blood (dysentery).     - Mechanism: Invades the villus of the large intestine and Peyer’s patches to trigger inflammation. It does not perforate the intestine or invade the blood.     - Treatment: Fluid replacement, ciprofloxacin, and sulfatrimethoprim.

The Enteric Yersinia Pathogens and Yersinia pestis (Plague)

• Enteric Yersinia:     - Y. enterocolitica: Found in animals, fish, water; enters small intestinal mucosa. Inflammation can mimic appendicitis.     - Y. pseudotuberculosis: Similar to Y. enterocolitica but with more lymph node involvement.
• Yersinia pestis (The Plague):     - Morphology: Tiny, gram-negative rod with unusual bipolar staining and a capsule.     - Virulence Factors: Capsular/envelope proteins (anti-phagocytic), coagulase, endotoxin, and murine toxin.
• Epidemiology and Cycle:     - Hosts: Found in $200$ mammalian species (rodents).     - Vectors: Fleas. Bacteria replicate in the flea's gut; coagulase cause blood clots that block the esophagus, making the flea ravenous.     - Types of Plague Cycles: Sylvatic (wild animals), Urban (domestic animals), and Human (accidental).
• Pathology:     - Infectious Dose (ID): $3-50$ bacilli.     - Bubonic Plague: Multiplies in flea bite; enters lymph causing necrosis and swelling of nodes (bubo) in groin or axilla.     - Septicemic Plague: Massive bacterial growth in blood; causes intravascular coagulation, subcutaneous hemorrhage, and purpura ("black plague").     - Pneumonic Plague: Localized to lungs; highly contagious and fatal without treatment.
• Diagnosis and Treatment:     - Diagnosis via history and aspiration of buboes.     - Treatment: Streptomycin, tetracycline, or chloramphenicol.

Haemophilus: The Blood-Loving Bacilli

• General Characteristics:     - Tiny, gram-negative pleomorphic rods.     - Growth Requirements: Fastidious; require factors from blood (hemin, NAD, or NADP). Must use chocolate agar for cultivation.
• Major Species:     - H. influenzae: Causes acute bacterial meningitis, epiglottitis, otitis media, sinusitis, and pneumonia.         - Hib Vaccine: Subunit vaccine used in children (often combined with TDaP). Introduction in 1990 dropped meningitis cases from $10$ per $100,000$ to less than $1$ by 1994.     - H. influenzae subtype aegyptius: Primary agent of pink eye (communicable conjunctivitis).     - H. ducreyi: Agent of chancroid (STD).     - H. parainfluenzae & H. aphrophilus: Normal oral flora; can be involved in infective endocarditis.