Microbiology- Clinical Bacteriology (Gram -) Flashcards
Gram-negative lab algorithm
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Neisseria Characteristics
Gram ⊝ diplococci. Metabolize glucose and produce IgA proteases. Contain lipooligosaccharides (LOS) with strong
endotoxin activity. N gonorrhoeae is often intracellular (within neutrophils).
Gonococci Characteristics, Clinical features, treatment
No polysaccharide capsule, Maltose not fermented, No vaccine due to antigenic variation of pilus proteins, Sexually or perinatally transmitted.
Causes gonorrhea, septic arthritis, neonatal conjunctivitis (2–5 days after birth), pelvic inflammatory disease (PID), and Fitz-Hugh– Curtis syndrome.
Treatment: ceftriaxone (+ azithromycin or doxycycline, for possible chlamydial coinfection).
Meningococci Characteristics, Clinical features, treatment
Polysaccharide capsule, Maltose fermentation, Vaccine (type B vaccine not widely available), Transmitted via respiratory and oral secretions.
Causes meningococcemia with petechial hemorrhages and gangrene of toes, meningitis, Waterhouse-Friderichsen syndrome (adrenal insufficiency, fever, DIC,
shock).
Treatment: ceftriaxone or penicillin G
Haemophilus influenzae Clinical features and treatment
HaEMOPhilus causes Epiglottitis (endoscopic appearance, can be “cherry red” in children; “thumb sign” on lateral neck x-ray), Meningitis, Otitis media, and Pneumonia.
Treatment: amoxicillin +/− clavulanate for mucosal infections; ceftriaxone for meningitis; rifampin prophylaxis for close contacts.
Haemophilus influenzae Characteristics, transmission, nontypeable strains, vaccine
Small gram ⊝ (coccobacillary) rod. Aerosol transmission. Nontypeable (unencapsulated) strains are the most common cause of mucosal infections (otitis media, conjunctivitis, bronchitis) as well as invasive infections since the vaccine for capsular type b was introduced.
Produces IgA protease.
Vaccine contains type b capsular polysaccharide (polyribosylribitol phosphate) conjugated to diphtheria toxoid or other protein. Given between 2 and 18 months of age.
Bordetella pertussis Clinical features
Catarrhal—low-grade fevers, Coryza.
Paroxysmal—paroxysms of intense cough followed by inspiratory “whooP” (“whooping cough”), posttussive vomiting.
Convalescent—gradual recovery of chronic cough.
Prevented by Tdap, DTaP vaccines. May be mistaken as viral infection due to lymphocytic infiltrate resulting from immune response.
Legionella pneumophila Characteristics, Transmission, Treatment
Gram stains poorly—use silver stain. Grow on charcoal yeast extract medium with iron and cysteine. Detected by
presence of antigen in urine. Labs may show hyponatremia.
Aerosol transmission from environmental water source habitat.
Treatment: macrolide or quinolone.
Legionnaires’ disease
Pontiac fever
severe pneumonia (often unilateral and lobar A ), fever, GI and CNS symptoms. Common in smokers and in chronic lung disease.
mild flu-like syndrome.
Treatments Pseudomonas aeruginosa
Include “CAMPFIRE” drugs:
Carbapenems
Aminoglycosides
Monobactams
Polymyxins (eg, polymyxin B, colistin)
Fluoroquinolones (eg, ciprofloxacin, levofloxacin)
ThIRd- and fourth-generation cephalosporins (eg, ceftazidime, cefepime).
Extended-spectrum penicillins (eg, piperacillin, ticarcillin)
PSEUDOMONAS is associated with:
Pneumonia, Sepsis, Ecthyma gangrenosum, UTIs, Diabetes, Osteomyelitis, Mucoid polysaccharide capsule, Otitis externa (swimmer’s ear), Nosocomial infections (eg,
catheters, equipment), Addicts (drug abusers), Skin infections (eg, hot tub folliculitis, wound infection in burn victims).
Pseudomonas aeruginosa Produces PEEP:
Phospholipase C (degrades cell membranes); Endotoxin (fever, shock); Exotoxin A (inactivates EF-2); Pigments: pyoverdine and pyocyanin
Ecthyma gangrenosum
Rapidly progressive, necrotic cutaneous lesion caused by
Pseudomonas bacteremia. Typically seen in immunocompromised patients.
Pseudomonas aeruginosa Characteristics
Aeruginosa—aerobic; motile, gram ⊝ rod. Nonlactose
fermenting. Oxidase ⊕. Frequently found in water. Has a grape-like odor.
Salmonella vs Shigella
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typhoid fever
(rose spots on abdomen, constipation, abdominal
pain, fever); treat with ceftriaxone or fluoroquinolone
Yersinia enterocolitica
Gram ⊝ rod. Usually transmitted from pet feces (eg, puppies), contaminated milk, or pork.
Causes acute diarrhea or pseudoappendicitis (right lower abdominal pain due to mesenteric adenitis and/ or terminal ileitis).
Lactose-fermenting enteric bacteria
Lactose is key. Test with MacConKEE’S agar.
Fermentation of lactose pink colonies on MacConkey agar. Examples include Citrobacter, Klebsiella, E coli, Enterobacter, and Serratia (weak fermenter).
Escherichia coli virulence factors
Gram ⊝ rod.
E coli virulence factors: fimbriae—cystitis and pyelonephritis (P-pili); K capsule— pneumonia, neonatal meningitis; LPS endotoxin—septic shock.
Escherichia coli:
Enteroinvasive
EIEC is Invasive; dysentery. Clinical manifestations similar to Shigella.
Escherichia coli:
Enterotoxigenic
ETEC; Traveler’s diarrhea (watery).
Escherichia coli:
Enteropathogenic
Diarrhea, usually in children (think EPEC and Pediatrics).
Escherichia coli:
Enterohemorrhagic
O157:H7
Dysentery (toxin alone causes necrosis and
inflammation).
Does not ferment sorbitol (vs other E coli).
Hemorrhagic, Hamburgers, Hemolytic-uremic syndrome.
hemolytic-uremic syndrome:
triad of anemia, thrombocytopenia, and acute renal failure due to microthrombi forming on damaged endothelium.
Klebsiella Clinical features
5 A’s of KlebsiellA: Aspiration pneumonia Abscess in lungs and liver Alcoholics DiAbetics “CurrAnt jelly” sputum
Campylobacter jejuni Clinical features
Major cause of bloody diarrhea, especially in children.
Fecal-oral transmission through person-to-person contact or via ingestion of undercooked contaminated poultry or meat, unpasteurized milk. Contact with infected animals (dogs, cats, pigs) is also a risk factor.
Common antecedent to Guillain-Barré syndrome and reactive arthritis.
Vibrio cholerae Clinical features
Produces profuse rice-water diarrhea via enterotoxin that permanently activates Gs, cAMP.
Sensitive to stomach acid (acid labile); requires large inoculum (high ID50) unless host has gastric acidity.
Transmitted via ingestion of contaminated water or uncooked food (eg, raw shellfish). Treat promptly with oral rehydration solution.
Helicobacter pylori Characteristics, Clinical features, Treatment
Curved, flagellated (motile), gram ⊝ rod that is triple ⊕: catalase ⊕, oxidase ⊕, and urease ⊕ (can use urea breath test or fecal antigen test for diagnosis).
Colonizes mainly antrum of stomach; causes gastritis and peptic ulcers (especially duodenal). Risk factor for peptic ulcer disease, gastric adenocarcinoma, and MALT lymphoma.
Most common initial treatment is triple therapy: Amoxicillin (metronidazole if penicillin allergy) + Clarithromycin + Proton pump inhibitor; Antibiotics Cure Pylori.
