(A) Consumption of contaminated food (B) Direct contact with fomite (C) Arthropod vector (D) Respiratory route (E) Sexual contact

Transcription

1 A 33-year-old man presents to the emergency department with a fever of F, facial palsy, headache, and malaise. A circular maculopapular rash was identified on the patients left shoulder; the patient was unaware of the rash. The patient likely acquired the above infection via which of the following routes? (A) Consumption of contaminated food (B) Direct contact with fomite (C) Arthropod vector (D) Respiratory route (E) Sexual contact Answer: C. The causal agent is Borrelia burgdorferi, and the disease is known as Lyme disease. The clues are facial palsy, rash, fever and malaise. Borrelia is spread by ticks. A 40-year-old homeless man presents to the emergency department with fever and night sweats, coughing up blood. Acid-fast bacilli are identified in his sputum. Which of the following virulence factors allows the causal agent to inhibit phagosome-lysosome fusion to survive intracellularly? (A) Cord factor (B) Calcium dipicolinate (C) Peptidoglycan (D) Sulfatides (E) Tuberculin Answer: D. The causal agent is Mycobacterium tuberculosis. The clues are coughing up blood, acid-fast bacilli, and homeless. Sulfatides are sulfolipids which hydrolyze to form sulfuric acid. The acidic ph of the M. tuberculosis-containing phagosome acts to stop lysosomal fusion. Cord factor (choice A) is responsible for serpentine growth in vitro. Calcium dipicolinate (choice B) is a component of endospores. Peptidoglycan (choice C) is a cell wall component. Tuberculin (choice E) is a surface protein, which is not involved in protection from phagosome-lysosome fusion. A homeless, malnourished chronic alcoholic presents with severe headache and dyspnea. Physical examination reveals a disheveled male with poor hygiene. His temperature is 41 C, blood pressure is 110/78 mm Hg, and his pulse is 96/ minute and regular. Auscultation of the chest reveals absence of breath sounds over the left middle lung fields. A chest x-ray confirms left lobar pneumonia. Sputum stain reveals partially acid-fast bacilli with branching rods. Which of the following agents is the most likely cause? (A) Mycobacterium avium-intracellulare (B) Mycobacterium kansasii (C) Mycobacterium leprae (D) Mycobacterium tuberculosis (E) Nocardia asteroides Answer: E. Partially acid-fast branching rods in a patient with lobar p monia suggests Nocardia. All the other agents listed are acid-fast bacilli, not branching rods.

2 Chapter 2 l Medically Important Bacteria GENUS: MYCOBACTERIUM Genus 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 tuberculosis Distinguishing Features l Auramine-rhodamine staining bacilli (fluorescent apple green); no antibody 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 temperature Key Vignette Clues Mycobacterium 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 217

3 Section II l Microbiology Reservoir human lungs Transmission respiratory droplets Figure II-2-7. Cord Factor 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 hypersensitivity 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 macrophages 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 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 poverty, 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 218

4 Chapter 2 l Medically Important Bacteria 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 mycobacteria) 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 organisms 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 Summary of MOTTS Organism Disease Transmission Clinical Presentation Treatment M. avium-intracellulare M. kansasii Pulmonary, Gastrointestinal, Disseminated Respiratory, Ingestion AIDS patients, cancer, chronic lung disease AIDS patients prophylaxis <50 CD4+ cells/mm 3 Macrolide plus ethambutol M. scrofulaceum Lymphadenitis Contaminated water sources M. marinum Soft tissue infections fish tank granuloma Abrasions Definition of abbreviation: INH, isonazid; LN, lymph node. Solitary cervical LN in kids Cutaneous granulomas in tropical fish enthusiasts Surgery INH Rifampin or ethambutol 219

5 Section II l Microbiology Note l M. tuberculosis in HIV patient with normal count or low CD4 count (disseminated) l MAI only in late HIV patient with low CD4 count Mycobacterium leprae Distinguishing 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 Transmission nasal discharge from untreated lepromatous leprosy patients Pathogenesis l Obligate intracellular parasite l Cooler parts of body, e.g., skin, mucous membranes, and peripheral nerves Key Vignette Clues Leprosy Disease(s) leprosy: a continuum of disease, which usually starts out with an indeterminate stage called borderline l Acid fast bacilli in punch biopsy l Immigrant patient with sensory loss in extremities Table II Two Extreme Forms of Leprosy Tuberculoid Lepromatous Cell-mediated immune system Strong CMI (Th1) Weak CMI (Th2) Lepromin skin test Lepromin test + Lepromin test Number of organisms in tissue Damage from Number of lesions and other symptoms Low Immune response (CMI killing infected cells) Granuloma formation nerve enlargement/ damage Loss of sensation burns and trauma Fewer lesions: macular; nerve enlargement, paresthesia B o r d e r l i n e High (foam cells totally filled) Large number of intracellular organisms Nerve damage from overgrowth of bacteria in cells Loss of sensation burns and trauma Numerous lesions becoming nodular; loss of eyebrows; destruction of nasal septum Paresthesia Leonine 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 Treatment multiple-drug therapy with dapsone and rifampin, with clofazimine added for lepromatous Prevention dapsone for close family contacts 220

6 Section II l Microbiology Note Comparative Microbiology l Invasive bacteria: PMN in stool: Shigella, Salmonella, Campylobacter, EIEC. l Toxigenic bacteria: ETEC, V. cholera, Cl. perfringens, EHEC. GENUS: SHIGELLA Genus 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 Key Vignette Clues Shigella l Patient with acute bloody diarrhea and fever l Gram ( ) bacilli, which are nonmotile, nonlactose fermenters, do not produce H 2 S Shigella Species Distinguishing Features l Gram-negative rods, nonmotile Reservoir human colon only (no animal carriers) Transmission fecal-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 laterally 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; diarrhea 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 240

7 Chapter 2 l Medically Important Bacteria 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) 241

8 Section II l Microbiology GENUS: VIBRIO Genus 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 Key Vignette Clues Vibrio 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 Vibrio cholerae Distinguishing 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 (>10 7 organisms), if stomach acid is normal Pathogenesis Disease 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 (G s alpha) activating adenylate cyclase increased camp efflux of Cl and H 2 O (persistent activation of adenylate cyclase) l Cholera Rice water stools, tremendous fluid loss Hypovolemic shock if not treated Diagnosis l Culture stool on TCBS l Oxidase positive 234

9 Chapter 2 l Medically Important Bacteria 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 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 spreading; difficult to treat Tetracycline; third-generation cephalosporins 235

10 Section II l Microbiology GENUS: BORRELIA Genus 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) Key Vignette Clues Borrelia 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 Borrelia burgdorferi Reservoirs 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 Disease l B. burgdorferi invades skin and spreads via the bloodstream to involve primarily the heart, joints, and central nervous system. l Arthritis is caused by immune complexes. l Lyme disease (#1 tick-borne disease in the U.S.) Stage 1: Early localized (3 days to 1 month) Stage 2: Early disseminated (days to weeks later) Organism spreads hematogenously Stage 3: Late persistent (months to years) Target rash Fatigue Chills and fever Headache Muscle and joint pain Swollen lymph nodes Secondary annular skin lesions Bell palsy, headache, meningitis, extreme fatigue, conjunctivitis, palpitations, arrhythmias, myocarditis, pericarditis Arthritis, most common in knees, immune complex-mediated Diagnosis l Serodiagnosis by ELISA negative early l Western blot for confirmation 256

11 Chapter 2 l Medically Important Bacteria 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. 257