Gram stain

N. meningitidis(“meningococcus”) has a polysaccharide capsule. Lives in nasopharynx and causes disease when entering bloodstream (meningitis, sepsis). Releases endotoxin causing hemorrhage (petechial rash), spiking fever, chills, myalgia. Can cause adrenal hemorrhage (Waterhouse-Friderichsen syndrome: adrenal insufficiency). Infants 6 months – 2 years, army recruits, dorm dwellers at risk.

Grouping of gram-negative bacilli with enhanced ability to cause infective endocarditis (IE). Includes Haemophilus parainfluenzae, Haemophilus aphrophilus, Haemophilus paraphrophilus, Actinobacillus actinomycetemcomitans, Cardiobacterium hominis, Eikenella corrodens, Kingellaspecies. Responsible for ~3% of native valve IE. Most common cause in persons who do not abuse IV drugs.

Causes nosocomial sepsis (second most common after E. coli), Foley catheter UTIs, pneumonia in aspiration-prone patients (alcoholic, intubated, debilitated). Pneumonia forms cavitations; sputum is classically currant jelly colored.

Normally present in GI tract; virulent strains via DNA exchange can cause diarrhea (in developing world infants, immunocompromised, travelers). Three types: enterotoxigenic (ETEC), enteroinvasive (EIEC), enterohemorrhagic (EHEC). E. coli most common UTI cause, affects women/Pt with Foley catheters. Neonatal E. coli meningitis common. Most common gram-negative sepsis in hospitalized patients; nosocomial pneumonia.

Three species: S. typhi(humans), S. cholerae-suis, S. enteritidis(animal GI tracts). Spread through contaminated food/water. S. typhi invades intestinal epithelium, then lymph/seed multiple organs. Typhoid fever: fever, diarrhea, headache, abdominal pain, inflamed organs (e.g., spleen). Light-skinned may show transient salmon-colored belly rash.

Fecal-oral transmission (contaminated water, poor hand hygiene), mainly in preschools/nursing homes. Bacteria attach to intestinal epithelium, invade, release Shiga toxin (inhibits protein synthesis, causes cell death). Results in inflammatory response, blood-and-pus diarrhea, abdominal pain.

Nonencapsulated (“untypable”) H. flu causes otitis media in children, upper respiratory infections in adults with chronic lung disease. Capsule limits invasiveness. Routine Hib vaccine has greatly reduced H. influenzae meningitis, acute epiglottitis, septic arthritis.

Tends to infect sick/immunocompromised; very resistant to antibiotics. Appears blue-green, sweet grape smell. Causes problems in burns, IE in IV drug abusers, pneumonia (immunocompromised, cystic fibrosis), sepsis, otitis externa/media (esp. diabetic), UTI/pyelonephritis, osteomyelitis in IV drug abusers/children.

Campylobacter jejuni—one of three leading causes of gastroenteritis (others: Rotavirus, ETEC), mostly in children. Reservoir is animals; acquired by fecal-oral route/unpasteurized milk. Prodrome: fever, headache, bloody diarrhea, abdominal cramps. Associated with Guillain-Barre syndrome.

Streptococcus pneumoniae has no Lancefield antigen, appears as lancet-shaped cocci in pairs (diplococci), and contains a polysaccharide capsule. These organisms are α-hemolytic. S. pneumoniaeis a common cause of pneumonia and meningitis in adults and of otitis media and conjunctivitis in children. The pneumococcal vaccine should be given to immunocompromised patients, older adults, those with HIV, and asplenic patients.

Heterogeneous group of streptococci not defined by a single Lancefield antigen but share common features: gastrointestinal tract location (especially saliva), α-hemolytic. Cause: 1. Dental caries; 2. Subacute (slow-onset) bacterial endocarditis, especially post-dental procedures and on damaged/prosthetic valves (acute endocarditis usually staphylococcal); 3. Brain/abdominal abscesses.

Lancefield group A strep, β-hemolytic. Multiple virulence factors produce disease by direct invasion, toxin release, or delayed antibody response. Can cause pharyngitis, skin infection, scarlet fever, toxic shock syndrome, rheumatic fever, and acute post-streptococcal glomerulonephritis.

Lancefield group B Streptococcus(group B strep, or GBS), β-hemolytic. Lives in the vagina of 25% of women, associated with newborn infection during birth; neonatal meningitis, pneumonia, and sepsis most common. Pregnant women tested for GBS at 35-37 weeks and receive prophylactic penicillin if positive. GBS also causes sepsis in pregnant women, patients with medical co-morbidities, older adults.

Group D Streptococcus; bacteremia highly associated with colon cancer.

Enterococcus faecalis and Enterococcus faecium are normal bowel inhabitants (grow well in bile) and commonly cause infections in hospitalized patients: UTI, biliary infections, endocarditis, wound infections, bacteremia/sepsis (IV catheters). May show resistance to VRE.

Present in nasal cavity of 30% of individuals, especially ill/hospital staff; spread by contact, reduced by handwashing. Most virulent staph species; survives long. Surface adhesion proteins (coagulase, protein A, lipase) contribute to virulence. Local skin infection may lead to furuncle/abscess, then invade organs via lymphatics/blood. Penicillin-resistant due to penicillinase. MRSA: resistant to beta-lactams, treat with vancomycin. MRSA may be >50% of cases.

Inhabits normal skin; frequent blood culture contaminant. Forms biofilm (polysaccharide scaffold), allowing adherence to synthetic materials (frequent on Foley catheters, IV lines, mechanical valves, stents, prosthetic joints).

Causes pseudomembranous enterocolitis, usually within a month after broad-spectrum antibiotic use (e.g. clindamycin, ampicillin), which reduces normal GI flora and allows overgrowth. Symptoms: abdominal cramps, diarrhea, fever due to toxin release. Diagnosis by toxin detection in stool. Treat with metronidazole (oral or IV) or vancomycin (oral only; IV vancomycin is ineffective for colitis).

Gram-positive, beaded, filamentous anaerobe part of normal oral flora; causes actinomycosis. Erodes oral/GI mucosa, creates draining sinus tracts. Pus from abscesses contains yellow “sulfur” granules (microcolonies of A. israelii). Treat with penicillin G and abscess drainage.