Microbiology- Clinical Bacteriology (Gram +) Flashcards
Gram-positive lab algorithm
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Gram-positive cocci antibiotic tests:
Staphylococci
NO StRESs.
NOvobiocin—Saprophyticus is Resistant; Epidermidis is Sensitive.
Gram-positive cocci antibiotic tests:
Streptococci
OVRPS (overpass).
Optochin—Viridans is Resistant; Pneumoniae is Sensitive.
B-BRAS.
Bacitracin—group B strep are Resistant; group A strep are Sensitive.
Gram-positive cocci antibiotic tests:
α-hemolytic bacteria
Gram ⊕ cocci. Partial reduction of hemoglobin causes greenish or brownish color without clearing around growth on blood agar.
Streptococcus pneumoniae (catalase ⊝ and optochin sensitive)
Viridans streptococci (catalase ⊝ and optochin resistant)
Gram-positive cocci antibiotic tests:
β-hemolytic bacteria
Gram ⊕ cocci. Complete lysis of RBCs clear area surrounding colony on blood agar.
Staphylococcus aureus (catalase and coagulase ⊕)
Streptococcus pyogenes—group A strep (catalase ⊝ and bacitracin sensitive)
Streptococcus agalactiae—group B strep (catalase ⊝ and bacitracin resistant)
Staphylococcal toxic shock syndrome (TSS)
fever, vomiting, rash, desquamation, shock, end-organ failure. TSS results in AST, ALT, bilirubin. Associated with prolonged use of vaginal tampons or nasal packing.
S aureus food poisoning
due to ingestion of preformed toxin short incubation period (2–6 hr) followed by nonbloody diarrhea and emesis. Enterotoxin is heat stable not destroyed by cooking.
MRSA (methicillin-resistant S aureus)
Important cause of serious nosocomial and community-acquired infections; resistant to methicillin and nafcillin because of altered penicillin-binding protein.
S. aeureus Inflammatory disease
skin infections, organ abscesses, pneumonia (often after
influenza virus infection), endocarditis, septic arthritis, and osteomyelitis.
S. aeureus Toxin-mediated disease
toxic shock syndrome (TSST-1), scalded skin syndrome
exfoliative toxin), rapid-onset food poisoning (enterotoxins
Staphylococcus epidermidis Disease
Normal flora of skin; contaminates blood cultures.
Infects prosthetic devices (eg, hip implant, heart valve) and IV catheters by producing adherent biofilms.
Staphylococcus saprophyticus Disease
Normal flora of female genital tract and perineum.
Second most common cause of uncomplicated UTI in young women (most common is E coli).
Streptococcus pneumoniae Disease
Most common cause of: Meningitis Otitis media (in children) Pneumonia Sinusitis
Viridans group streptococci Disease
Streptococcus mutans and S mitis cause dental caries.
S sanguinis makes dextrans that bind to fibrinplatelet
aggregates on damaged heart valves, causing subacute bacterial endocarditis.
*Normal flora of the oropharynx.
Streptococcus pyogenes (group A streptococci) Disease
Cause:
Pyogenic—pharyngitis, cellulitis, impetigo (“honey-crusted” lesions), erysipelas.
Toxigenic—scarlet fever, toxic shock–like syndrome, necrotizing fasciitis.
Immunologic—rheumatic fever, glomerulonephritis.
Scarlet fever
blanching, sandpaper-like body rash, strawberry tongue, and circumoral pallor in the setting of group A streptococcal pharyngitis (erythrogenic toxin ⊕).
Streptococcus pyogenes (group A streptococci) Tests
Bacitracin sensitive, β-hemolytic, pyrrolidonyl arylamidase (PYR) ⊕.
Hyaluronic acid capsule and M protein inhibit phagocytosis. Antibodies to M protein can give rise to rheumatic fever.
ASO titer or anti-DNas B antibodies indicate recent S pyogenes infection.
Streptococcus agalactiae (group B streptococci) Disease
colonizes vagina; causes pneumonia, meningitis, and sepsis, mainly in babies.
Hippurate test ⊕. PYR ⊝.
Streptococcus agalactiae (group B streptococci) screening
Screen pregnant women at 35–37 weeks of gestation with rectal and vaginal swabs.
Patients with ⊕ culture receive intrapartum
penicillin prophylaxis.
Streptococcus bovis Disease
Bovis in the blood = cancer in the colon.
colonizes the gut. S gallolyticus (S bovis biotype 1) can cause bacteremia and subacute endocarditis and is associated with colon cancer.
Enterococci (E faecalis and E faecium) Disease
Catalase ⊝, PYR ⊕, variable hemolysis.
Normal colonic flora that are penicillin G resistant and cause UTI, biliary tract infections, and subacute endocarditis (following GI/GU procedures).
VRE (vancomycin-resistant enterococci) are an important cause of nosocomial infection.
Bacillus anthracis Characteristics
Gram ⊕, spore-forming rod that produces anthrax toxin.
The only bacterium with a polypeptide capsule (contains d-glutamate).
Colonies show a halo of projections, sometimes referred to as “medusa head” appearance.
Cutaneous anthrax
Painless papule surrounded by vesicles, ulcer with black eschar (painless, necrotic), uncommonly progresses to bacteremia and death.
Pulmonary anthrax
Inhalation of spores, flu-like symptoms that rapidly progress to fever, pulmonary hemorrhage, mediastinitis, and shock. Also known as woolsorter’s disease. CXR may show widened mediastinum.
Bacillus cereus Characteristics
Gram ⊕ rod. Causes food poisoning. Spores survive cooking rice (also known as reheated rice syndrome).
Keeping rice warm results in germination of spores and enterotoxin formation.
Nausea and vomiting within 1–5 hr. Caused by cereulide, a preformed toxin.
Diarrheal type causes watery, nonbloody diarrhea and GI pain within 8–18 hr.
Clostridia (with exotoxins)
Gram ⊕, spore-forming, obligate anaerobic rods.
C tetani Clinical characteristics
Causes spastic paralysis, trismus (lockjaw), risus sardonicus (raised eyebrows and open grin), opisthotonos (spasms of spinal extensors).
Prevent with tetanus vaccine. Treat with antitoxin +/− vaccine booster, antibiotics, diazepam (for muscle spasms), and wound debridement.
C botulinum Clinical characteristics
Symptoms of botulism (the 4 D’s): Diplopia, Dysarthria, Dysphagia, Dyspnea.
Botulinum is from bad bottles of food, juice, and
honey (causes a descending flaccid paralysis).
Treat with human botulinum immunoglobulin.
Local botox injections used to treat…
focal dystonia, achalasia, and muscle spasms. Also
used for cosmetic reduction of facial wrinkles.
C perfringens Clinical characteristics
Produces α toxin (lecithinase, a phospholipase) that can cause myonecrosis (gas gangrene; presents as soft tissue crepitus) and hemolysis.
C difficile Clinical characteristics
Often 2° to antibiotic use, especially clindamycin or ampicillin.
Toxin A, an enterotoxin, binds to brush border of gut and alters fluid secretion.
Toxin B, a cytotoxin, disrupts cytoskeleton via actin depolymerization. Both toxins lead to diarrhea pseudomembranous colitis.
C difficile Diagnosis and Treatment
Diagnosed by PCR or antigen detection of one
or both toxins in stool.
Treatment: metronidazole or oral vancomycin. For
recurrent cases, consider repeating prior regimen, fidaxomicin, or fecal microbiota transplant.
Corynebacterium diphtheriae Clinical Features
transmitted via respiratory droplets. Causes diphtheria
Symptoms include pseudomembranous pharyngitis (grayish-white membrane) with lymphadenopathy, myocarditis, and arrhythmias.
Corynebacterium diphtheriae Diagnosis
Lab diagnosis based on gram ⊕ rods with metachromatic (blue and red) granules and ⊕ Elek test for toxin.
Toxoid vaccine prevents diphtheria.
Listeria monocytogenes acquisition way
acquired by ingestion of unpasteurized dairy products and cold deli meats, via transplacental transmission, or by vaginal transmission during birth. Grows well at refrigeration temperatures.
Forms “rocket tails” (red in A ) via actin polymerization that allow intracellular movement and cellto-cell spread across cell membranes.
Listeria monocytogenes Clinical features and treatment
Can cause amnionitis, septicemia, and spontaneous abortion in pregnant women; granulomatosis infantiseptica; neonatal meningitis; meningitis in immunocompromised patients; mild, selflimited gastroenteritis in healthy individuals.
Treatment: ampicillin.
Nocardia Tests, Disease, Treatment
Aerobe, Acid fast (weak), Found in soil.
Causes pulmonary infections in immunocompromised (can mimic TB but with ⊝ PPD); cutaneous infections after
trauma in immunocompetent; can spread to CNS.
Treat with sulfonamides (TMP-SMX)
Treatment is a SNAP: Sulfonamides—Nocardia; Actinomyces—Penicillin
Actinomyces Tests, Disease, Treatment
Anaerobe, Not acid fast, Normal oral, reproductive, and GI flora.
Causes oral/facial abscesses that drain through sinus tracts; often associated with dental caries/ extraction and other maxillofacial trauma; forms yellow “sulfur granules”; can also cause PID with IUDs.
Treat with penicillin
Common Characteristics of actinomyces and Nocardia
Both are gram ⊕ and form long, branching filaments resembling fungi.
Disease:
M scrofulaceum
M marinum
- cervical lymphadenitis in children.
- hand infection in aquarium handlers.
M avium–intracellulare Disease and prophylaxis
causes disseminated, non-TB disease in AIDS; often resistant to multiple drugs.
Prophylaxis with azithromycin when CD4+ count < 50 cells/ mm3.
TB algorithm
Pag. 140
Leprosy (Hansen disease) Bacterium characteristics, diagnosis and treatment
Caused by Mycobacterium leprae, an acid-fast bacillus that likes cool temperatures (infects skin and superficial nerves—“glove and stocking” loss of sensation) and cannot be grown in vitro.
Diagnosed via skin biopsy or tissue PCR. Reservoir in United States: armadillos.
Treatment: dapsone and rifampin for tuberculoid form; clofazimine is added for lepromatous form.
Leprosy Tuberculoid
limited to a few hypoesthetic, hairless skin plaques; characterized by high cellmediated immunity with a largely Th1-type immune response and low bacterial load.
Leprosy Lepromatous
Presents diffusely over the skin, with leonine (lion-like) facies, and is communicable (high bacterial load); characterized by low cell-mediated immunity with a
humoral Th2 response.
Lepromatous form can be lethal.
