A33-year-oldmanpresentstotheemergencydepartmentwithafeverof102.5°F,facialpalsy,headache,andmalaise.Acircularmaculopapularrashwasidentifiedonthepatientsleftshoulder;thepatientwasunawareoftherash.Thepatientlikelyacquiredtheaboveinfectionviawhichofthefollowingroutes?(A)Consumptionofcontaminatedfood(B)Directcontactwithfomite(C)Arthropodvector(D)Respiratoryroute(E)SexualcontactAnswer:C.ThecausalagentisBorreliaburgdorferi,andthediseaseisknownasLymedisease.Thecluesarefacialpalsy,rash,feverandmalaise.Borreliaisspreadbyticks.A40-year-oldhomelessmanpresentstotheemergencydepartmentwithfeverandnightsweats,coughingupblood.Acid-fastbacilliareidentifiedinhissputum.Whichofthefollowingvirulencefactorsallowsthecausalagenttoinhibitphagosome-lysosomefusiontosurviveintracellularly?(A)Cordfactor(B)Calciumdipicolinate(C)Peptidoglycan(D)Sulfatides(E)TuberculinAnswer:D.ThecausalagentisMycobacteriumtuberculosis.Thecluesarecoughingupblood,acid-fastbacilli,andhomeless.Sulfatidesaresulfolipidswhichhydrolyzetoformsulfuricacid.TheacidicpHoftheM.tuberculosis-containingphagosomeactstostoplysosomalfusion.Cordfactor(choiceA)isresponsibleforserpentinegrowthinvitro.Calciumdipicolinate(choiceB)isacomponentofendospores.Peptidoglycan(choiceC)isacellwallcomponent.Tuberculin(choiceE)isasurfaceprotein,whichisnotinvolvedinprotectionfromphagosome-lysosomefusion.Ahomeless,malnourishedchronicalcoholicpresentswithsevereheadacheanddyspnea.Physicalexaminationrevealsadisheveledmalewithpoorhygiene.Histemperatureis41°C,bloodpressureis110/78mmHg,andhispulseis96/minuteandregular.Auscultationofthechestrevealsabsenceofbreathsoundsovertheleftmiddlelungfields.Achestx-rayconfirmsleftlobarpneumonia.Sputumstainrevealspartiallyacid-fastbacilliwithbranchingrods.Whichofthefollowingagentsisthemostlikelycause?(A)Mycobacteriumavium-intracellulare(B)Mycobacteriumkansasii(C)Mycobacteriumleprae(D)Mycobacteriumtuberculosis(E)NocardiaasteroidesAnswer:E.Partiallyacid-fastbranchingrodsinapatientwithlobarpmoniasuggestsNocardia.Alltheotheragentslistedareacid-fastbacilli,notbranchingrods.
Chapter 2 l Medically Important Bacteria
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GENUS: MYCOBACTERIUMGenus Features
l Acid fast rods with a waxy cell wall
l Obligate aerobe
l Cell wall
− Unique: high concentration of lipids containing long chain fatty acids called mycolic acids
− Wall makes mycobacteria highly resistant to:
º Desiccation
º Many chemicals (including NaOH used to kill other bacteria in sputa before neutralizing and culturing)
l Sensitive to UV
Species of Medical Importance
l M. tuberculosis
l M. leprae
l M. avium-intracellulare
l M. kansasii
l M. scrofulaceum
l M. marinum
Mycobacterium tuberculosisDistinguishing Features
l Auramine-rhodamine staining bacilli (fluorescent apple green); no anti-body involved (sensitive but not specific)
l Acid fast
l Aerobic, slow growing on Lowenstein-Jensen medium; new culture systems (broths with palmitic acid) faster
l Produces niacin
l Produces a heat-sensitive catalase
− Catalase negative at 68.0°C (standard catalase test)
− Catalase active at body temperatureKey Vignette CluesMycobacterium tuberculosis
l High-risk patient (poverty, HIV+, IV drug user)
l Chronic cough, weight loss
l Ghon complex
l Auramine-rhodamine staining, acid fast bacilli in sputum
l Produce niacin, heat-sensitive catalase
l Positive PPD
l Facultative intracellular
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Section II l Microbiology
Reservoirhuman lungs
Transmissionrespiratory droplets
Pathogenesis
l Facultative intracellular organism (most important)
l Sulfatides (sulfolipids in cell envelope)
− Inhibit phagosome-lysosome fusion, allowing intracellular survival (if fusion occurs, waxy nature of cell envelope reduces killing effect)
l Cord factor (trehalose dimycolate)
− Causes serpentine growth in vitro
− Inhibits leukocyte migration; disrupts mitochondrial respiration and oxidative phosphorylation
l Tuberculin (surface protein) along with mycolic acid → delayed hypersensi-tivity and cell-mediated immunity (CMI)
− Granulomas and caseation mediated by CMI
− No exotoxins or endotoxin; damage done by immune system
Disease(s)
l Primary pulmonary tuberculosis
− Organisms replicate in naive alveolar macrophages, killing the macro-phages until CMI is set up (Ghon focus)
− Macrophages transport the bacilli to the regional lymph node (Ghon complex) and most people heal without disease
− Organisms that are walled off within the Ghon complex remain viable unless treated
l Reactivational tuberculosis
− Erosion of granulomas into airways (high oxygen) later in life under conditions of reduced T-cell immunity leads to mycobacterial replication and disease symptoms
− Complex disease with the potential of infecting any organ system
− May disseminate (miliary TB)
Diagnosis
l Microscopy of sputum: screen with auramine-rhodamine stain (fluorescent apple-green); no antibody involved; very sensitive; if positive, confirm with acid fast stain
l PPD skin test (Mantoux): measure zone of induration at 48–72 hours; positive if:
− ≥5 mm in HIV+ or anyone with recent TB exposure; AIDS patients have reduced ability to mount skin test.
− ≥10 mm in high-risk population: IV drug abusers, people living in pov-erty, or immigrants from high TB area
− ≥15 mm in low-risk population
l Positive skin test indicates only exposure but not necessarily active disease.
l Quantiferon-TB Gold Test: measures interferon-gamma production when leukocytes exposed to TB antigens
l Slow-growing (3–6 weeks) colonies on Lowenstein-Jensen medium (faster new systems)
l Organisms produce niacin and are catalase-negative (68°C).
l No serodiagnosis
Figure II-2-7.Cord Factor
Chapter 2 l Medically Important Bacteria
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Treatment
l Multiple drugs critical to treat infection
l Standard observed short-term therapy for uncomplicated pulmonary TB (rate where acquired resistance <4%):
− First 2 months: rifampin + isoniazid + pyrazinamide + ethambutol (RIPE)
− Next 4 months: rifampin and isoniazid
l Ethambutol or streptomycin added for possible drug-resistant cases until susceptibility tests are back (if area acquired has >4% drug-resistant myco-bacteria)
Prevention
l Isoniazid taken for 9 months can prevent TB in persons with infection but no clinical symptoms.
l Bacille Calmette-Guérin (BCG) vaccine containing live, attenuated organ-isms may prevent disseminated disease; not used in U.S.
l UV lights or HEPA filters used to treat potentially contaminated air.
Mycobacteria Other than Tuberculosis (MOTTS)Distinguishing Features
l Atypical mycobacteria
l Noncontagious
l Found in surface waters, soil, cigarettes
l Commonly found in southeastern U.S.
Table II-2-16. Summary of MOTTS
Organism Disease Transmission Clinical Pre-sentation
Treatment
M. avium-intra-cellulare
Pulmonary, Gastrointestinal, Disseminated
Respiratory,Ingestion
AIDS pa-tients, cancer, chronic lung disease
AIDS patients pro-phylaxis <50 CD4+ cells/mm3
Macrolide plus eth-ambutol
M. kansasii
M. scrofulaceum Lymphadenitis Contaminated water sources
Solitary cervi-cal LN in kids
Surgery
M. marinum Soft tissue infec-tions “fish tank granu-loma”
Abrasions Cutaneous granulomas in tropical fish enthusiasts
INHRifampin or etham-butol
Definition of abbreviation: INH, isonazid; LN, lymph node.
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Section II l Microbiology
Mycobacterium lepraeDistinguishing Features
l Acid fast rods (seen in punch biopsy)
l Obligate intracellular parasite (cannot be cultured in vitro)
l Optimal growth at less than body temperature
Reservoir
l Human mucosa, skin, and nerves are only significant reservoirs
l Some infected armadillos in Texas and Louisiana
Transmissionnasal discharge from untreated lepromatous leprosy patients
Pathogenesis
l Obligate intracellular parasite
l Cooler parts of body, e.g., skin, mucous membranes, and peripheral nerves
Disease(s)—leprosy: a continuum of disease, which usually starts out with an indeter-minate stage called “borderline”
Table II-2-17. Two Extreme Forms of Leprosy
Tuberculoid
Borderline
Lepromatous
Cell-mediated im-mune system
Strong CMI (Th1) Weak CMI (Th2)
Lepromin skin test Lepromin test + Lepromin test –
Number of organ-isms in tissue
Low High (foam cells totally filled)
Damage from Immune response (CMI killing infected cells)Granuloma formation → nerve enlargement/damageLoss of sensation → burns and trauma
Large number of intracel-lular organismsNerve damage from over-growth of bacteria in cellsLoss of sensation → burns and trauma
Number of lesions and other symptoms
Fewer lesions: macular; nerve enlargement, paresthesia
Numerous lesions be-coming nodular; loss of eyebrows; destruction of nasal septumParesthesiaLeonine facies
Diagnosis
l Punch biopsy or nasal scrapings; acid fast stain
l Lepromin skin test is positive in the tuberculoid but not in the lepromatous form
l No cultures
Treatmentmultiple-drug therapy with dapsone and rifampin, with clofazimine added for lepromatous
Preventiondapsone for close family contacts
Notel M. tuberculosis in HIV patient with
normal count or low CD4 count (dis-seminated)
l MAI only in late HIV patient with low CD4 count
Key Vignette CluesLeprosy
l Acid fast bacilli in punch biopsy
l Immigrant patient with sensory loss in extremities
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Section II l Microbiology
GENUS: SHIGELLAGenus Features
l Gram-negative rod
l Enterobacteriaceae
l Non–lactose fermenters (colorless colonies on EMB or MacConkey)
l Nonmotile
Species of Medical Importance
l Shigella sonnei (most common in U.S.)
l Shigella flexneri
l Shigella dysenteriae (most severe disease)
l Shigella boydii
Shigella SpeciesDistinguishing Features
l Gram-negative rods, nonmotile
Reservoir—human colon only (no animal carriers)
Transmissionfecal-oral spread, person to person
Pathogenesis
l Endotoxin triggers inflammation.
l No H antigens
l Shigellae invade M cells (membrane ruffling and macropinocytosis), get into the cytoplasm, replicate, and then polymerize actin jet trails to go lat-erally without going back out into the extracellular milieu. This produces very shallow ulcers and rarely causes invasion of blood vessels.
l Shiga toxin:
– Produced by S. dysenteriae, type 1
– Three activities: neurotoxic, cytotoxic, enterotoxic
– AB component toxin is internalized in human cells; inhibits protein synthesis by clipping 60S ribosomal subunit
Disease(s)
l Enterocolitis/shigellosis (most severe form is dysentery)
– Few organisms required to start infection (1–10) (extremely acid resistant)
– 1–4 day incubation
– Organisms invade, producing bloody diarrhea.
– Fever (generally >101.0°F); lower abdominal cramps; tenesmus; diar-rhea first watery, then bloody; invasive but rarely causes septicemia; shallow ulcers
– Severity depends on the age of patient and the strain; S. dysenteriae type 1 with toxin most severe
Diagnosis—isolation from stool during illness and culture on selective media
Note
Comparative Microbiology
l Invasive bacteria: PMN in stool: Shi-gella, Salmonella, Campylobacter, EIEC.
l Toxigenic bacteria: ETEC, V. cholera, Cl. perfringens, EHEC.
Key Vignette CluesShigella
l Patient with acute bloody diarrhea and fever
l Gram (−) bacilli, which are nonmotile, nonlactose fermenters, do not produce H2S
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Treatment
l Mild cases: fluid and electrolyte replacement only
l Severe cases: antibiotics
l Resistance is mediated by plasmid-encoded enzymes.
l Many strains are ampicillin resistant.
Prevention—proper sanitation (sewage, clean drinking water, hand washing)
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Section II l Microbiology
GENUS: VIBRIOGenus Features
l Gram-negative curved rod with polar flagella
l Oxidase positive
l Vibrionaceae
l Growth on alkaline, but not acidic, media (TCBS, thiosulfate citrate bile salt sucrose medium)
Species of Medical Importance
l Vibrio cholerae
l Vibrio parahaemolyticus
l Vibrio vulnificus
Vibrio choleraeDistinguishing Features
l “Shooting star” motility inactivated by specific serum
Reservoir
l Human colon; no vertebrate animal carriers (copepods or shellfish may be contaminated by water contamination)
l Human carriage may persist after untreated infection for months after infection; permanent carrier state is rare.
Transmission
l Fecal-oral spread; sensitive to stomach acid
l Requires high dose (>107 organisms), if stomach acid is normal
Pathogenesis
l Motility, mucinase, and toxin coregulated pili (TCP) aid in attachment to the intestinal mucosa.
l Cholera enterotoxin (choleragen) similar to E. coli LT; ADP ribosylates (Gs alpha) activating adenylate cyclase → increased cAMP → efflux of Cl– and H2O (persistent activation of adenylate cyclase)
Disease
l Cholera
− Rice water stools, tremendous fluid loss
− Hypovolemic shock if not treated
Diagnosis
l Culture stool on TCBS
l Oxidase positive
Key Vignette CluesVibrio cholerae
l Patient with noninflammatory diarrhea
l Rice-water stool
l Dehydration
l Travel to endemic area
l Gram (−) curved rods, polar flagellae, oxidase (+)
l Alkaline growth
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Treatment
l Fluid and electrolyte replacement
l Doxycycline or ciprofloxacin shorten disease and reduce carriage
l Resistance to tetracycline reported
Prevention—proper sanitation; new vaccine
Other Vibrio Species
Table II-2-20. Additional Vibrio Species
Species Reservoir Transmission Disease Symptoms Treatment
V. parahaemolyticus Marine life Consumption of undercooked or raw seafood
Gastroenteritis Watery diarrhea with cramping and abdominal pain
Self-limiting
V. vulnificus Brackish water, oysters
Consumption of undercooked or raw seafood
Gastroenteritis As above As above
Swimming in brackish water, shucking oysters
Cellulitis Rapidly spread-ing; difficult to treat
Tetracycline; third-generation cephalosporins
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Section II l Microbiology
GENUS: BORRELIAGenus Features
l Larger spirochetes
l Gram negative
l Microaerophilic
l Difficult to culture
Species of Medical Importance—Borrelia burgdorferi (10 species responsible for human disease)
Borrelia burgdorferiReservoirs—white-footed mice (nymphs) and white-tailed deer (adult ticks)
Transmission
l By Ixodes (deer) ticks and nymphs; worldwide but in 3 main areas in the U.S.:
– Ixodes scapularis (I. dammini) in Northeast (e.g., Connecticut), Midwest (e.g., Wisconsin)
– Ixodes pacificus on West Coast (e.g., California)
− Late spring/early summer incidence
Pathogenesis
l B. burgdorferi invades skin and spreads via the bloodstream to involve pri-marily the heart, joints, and central nervous system.
l Arthritis is caused by immune complexes.
Disease
l Lyme disease (#1 tick-borne disease in the U.S.)
Stage 1: Early localized (3 days to 1 month)
Target rash
Stage 2: Early disseminated (days to weeks later)
Organism spreads hematogenously
Fatigue Chills and fever Headache Muscle and joint pain Swollen lymph nodes Secondary annular skin lesions
Stage 3: Late persistent (months to years)
Bell palsy, headache, meningitis, extreme fatigue, conjunctivitis, palpitations, ar-rhythmias, myocarditis, pericarditis
Arthritis, most common in knees, immune complex-mediated
Diagnosis
l Serodiagnosis by ELISA—negative early
l Western blot for confirmation
Key Vignette CluesBorrelia burgdorferi
l Patient with influenza-like symptoms and erythema migrans
l Spring/summer seasons
l Northeast, Midwest, West Coast
l Later—neurologic, cardiac, arthritis/arthralgias
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Treatment
l Doxycycline, amoxicillin, or azithromycin/clarithromycin (primary)
l Ceftriaxone for secondary
l Doxycycline or ceftriaxone for arthritis
Prevention
l DEET; avoid tick bites
l Vaccine (OspA flagellar antigen) not used in the U.S.
