bacterial disease (26-50)

Lyme Disease

• Etiology & Microbiology

  • Causative spirochete: Borrelia burgdorferi (occasionally B. mayonii in the U.S.).

  • Gram-negative, loosely coiled helical bacteria; requires Ixodes tick vector for life-cycle completion.

• Ecology / Source & Transmission

  • Reservoir: White-footed mice and other small mammals; deer are important for adult tick feeding but not competent reservoirs.

  • Vector: Black-legged tick (Ixodes scapularis in Eastern/Mid-west US, I. pacificus in West).

  • Human infection via nymph stage (≤2 mm); greatest risk in late spring–summer during outdoor activities (camping, hiking).

• Pathogenesis

  • Spirochete disseminates from bite site → immune-mediated inflammation in skin, joints, nervous system.

  • Cardinal lesion “erythema migrans” (“bull’s-eye” rash) from local multiplication.

• Clinical Manifestations

  1. Early Localized (days–weeks): single expanding rash, fever, headache, myalgias.

  2. Early Disseminated (weeks–months): multiple EM lesions, cranial nerve VII palsy, meningitis, carditis (AV block).

  3. Late (months–years): arthritis of large joints, chronic encephalopathy.

• Diagnosis

  • Clinical EM alone is diagnostic.

  • Two-tier serology: ELISA → Western blot; may be negative in first $\le 4$ weeks.

• Management

  • First line: doxycycline 100 mg bid × 10–21 days.

  • Alternatives (children <8 y, pregnancy): amoxicillin or cefuroxime.

  • Post-exposure prophylaxis: single 200 mg doxycycline within 72 h if tick attached ≥$36$ h in high-endemic county.

• Prevention

  • Personal: DEET repellents, permethrin-treated clothing, tick checks.

  • Vaccine (LYMErix) withdrawn 2002; none currently for public.

• Public-Health / Ethical Note

  • Debate over “chronic Lyme”; long-term antibiotics lack evidence → stewardship issue.

Melioidosis

• Agent: Burkholderia pseudomallei, Gram-negative bacillus, facultative intracellular.

• Environment: Endemic in SE Asia, Northern Australia; present in wet soils, stagnant water, rice paddies.

• Transmission: Percutaneous inoculation (bare feet, gardening), inhalation (monsoons, typhoons, hurricanes), ingestion of contaminated water.

• Risk Groups: Farmers, diabetics (≈$50\%$ of cases), alcohol use disorder, chronic kidney disease.

• Clinical Spectrum

  • Acute pneumonia ± septic shock; multifocal abscesses (liver, spleen, prostate) → nickname “Vietnamese time-bomb” for latent reactivation decades later.

• Diagnosis

  • Culture on Ashdown’s agar (wrinkled purple colonies).

• Treatment

  • Intensive phase: IV ceftazidime, meropenem, or imipenem ≥$14$ days.

  • Eradication phase: oral TMP-SMX ± doxycycline for $3–6$ mo to prevent relapse.

• Prophylaxis / Vaccine: None; lab workers require BSL-3 precautions (Category B biothreat).

Meningococcal Disease

• Pathogen: Neisseria meningitidis (A, B, C, W, X, Y serogroups).

• Epidemiology

  • Colonizes nasopharynx (≈$10$–$20\%$ carriage).

  • Outbreaks in dormitories, military barracks, Hajj pilgrimage (serogroup W historically).

• Pathogenesis: Capsular polysaccharide blocks complement → bacteremia → leptomeningeal invasion; endotoxin triggers $\text{IL-1}$, $\text{TNF-α}$ → vascular collapse.

• Clinical: Rapid triad: fever, headache, nuchal rigidity + petechial/purpuric rash; may progress to Waterhouse–Friderichsen (adrenal hemorrhage).

• Management

  • Empiric: IV ceftriaxone $\pm$ vancomycin.

  • Confirmed: high-dose IV penicillin G.

  • Chemoprophylaxis for contacts: rifampin $600$ mg bid × 2 d or single ciprofloxacin $500$ mg.

• Vaccines

  • Conjugate MenACWY (ages $11–12$ y + booster).

  • MenB (Trumenba / Bexsero) for $16–23$ y or high risk (complement deficiency).

• Public-Health Note: Time-to-antibiotic critical; every $1$ h delay ↑ mortality ≈$17\%$.

MRSA Infection

• Organism: Methicillin-resistant Staphylococcus aureus (mecA gene → PBP2a → $\beta$-lactam resistance).

• Types

  • HA-MRSA (health-care): multiresistant, SCCmec I-III.

  • CA-MRSA (community): PVL toxin, SCCmec IV/V, skin/soft-tissue outbreaks among athletes, military, MSM.

• Clinical: Furuncles, carbuncles, cellulitis, necrotizing pneumonia post-influenza.

• Therapy

  • Severe: vancomycin (target trough $15–20$ µg/mL) or linezolid, daptomycin (not for pneumonia).

  • Mild SSTI: TMP-SMX, clindamycin, doxycycline.

• Prevention: Contact precautions, chlorhexidine decolonization, antimicrobial stewardship.

• Ethical Issue: Balancing empiric broad coverage vs resistance promotion.

Mycoplasma Pneumonia (“Walking Pneumonia”)

• Organism: Mycoplasma pneumoniae; lacks cell wall → not visible on Gram stain, “fried-egg” colonies on Eaton agar.

• Transmission: Respiratory droplets, incubation $1–4$ wk; outbreaks in schools, barracks.

• Clinical Features

  • Dry hacking cough, low-grade fever, bullous myringitis; extrapulmonary: hemolytic anemia (cold agglutinins $\text{IgM}$ against $I$ antigen), Stevens–Johnson, encephalitis.

• Diagnosis: CXR often “worse than patient” (reticulonodular infiltrates); PCR or cold-agglutinin titer $\ge 1:64$.

• Treatment: Macrolide (azithro), doxycycline, or respiratory fluoroquinolone.

• No Vaccine; immunity incomplete → reinfection.

Necrotizing Fasciitis (NF)

• Causative Agents

  • Type I (polymicrobial): anaerobes + Enterobacterales + Strep spp.

  • Type II: Group A Streptococcus (GAS); Type III: Vibrio vulnificus; Type IV: fungal.

• Pathophysiology: Rapid spread along fascial planes, thrombosis of perforating vessels → ischemia → tissue necrosis; GAS produces streptolysin O, exotoxin B.

• Hallmarks: Pain out of proportion, “wooden” hard tissue, bullae, systemic toxicity.

• Management

  • Surgical debridement within <6 h.

  • Empiric antibiotics: carbapenem or $\beta$-lactam/$\beta$-lactamase inhibitor + clindamycin (to block toxin) + vancomycin.

  • Mortality $20–40\%$ even with therapy.

Nocardiosis

• Organism: Nocardia spp. (e.g., N. asteroides complex); weakly acid-fast, branching filaments.

• Ecology: Soil; aerosolized or traumatic inoculation.

• Risk: Cell-mediated immunity defects—long-term steroids, transplant, HIV.

• Clinical

  • Pulmonary nodules/cavitation mimicking TB; brain abscess in $\approx$$20\%$; cutaneous lymphocutaneous disease in gardeners.

• Diagnosis: Modified acid-fast (Fite) stain, culture takes $\ge 1$ wk.

• Treatment: High-dose TMP-SMX (10–20 mg/kg TMP) for $6–12$ mo; severe: add imipenem or amikacin.

Pertussis (Whooping Cough)

• Agent: Bordetella pertussis, Gram-negative coccobacillus.

• Virulence: Pertussis toxin (↑$cAMP$), filamentous hemagglutinin, tracheal cytotoxin.

• Stages

  1. Catarrhal (1–2 wk): rhinorrhea, mild cough—highest contagiousness.

  2. Paroxysmal (2–6 wk): fits of $\ge 15$ coughs → inspiratory “whoop,” post-tussive emesis, extremed lymphocytosis ($\ge 15$ 000/µL).

  3. Convalescent (months): gradual waning.

• Treatment: Azithromycin (5-day), clarithromycin, or TMP-SMX if >$1$ mo old.

• Prevention: DTaP (children) at $2,4,6,15–18$ mo & 4–6 y; Tdap booster at $11–12$ y & each pregnancy (27–36 wk gestation).

Plague

• Etiologic Agent: Yersinia pestis, Gram-negative bipolar “safety-pin” rod, facultative intracellular.

• Epidemiology: Rodent reservoir (ground squirrels, prairie dogs); flea vector Xenopsylla; endemic foci in SW US, Madagascar, Congo.

• Clinical Forms

  • Bubonic: painful buboes (groin, axilla), 60–90 % of cases.

  • Septicemic: DIC, gangrene (“Black Death”).

  • Pneumonic: fulminant pneumonia, person-to-person droplets; CFR $\approx 100\%$ if untreated within $24$ h.

• Diagnosis: Gram/Giemsa stain of bubo aspirate; F1 antigen PCR.

• Treatment: Streptomycin $1$ g IM bid or gentamicin; doxycycline or ciprofloxacin alternatives (10–14 d).

• Prevention: Live attenuated vaccine for lab/high-risk; droplet isolation 48 h after antibiotics.

Pseudomonas Infection

• Organism: Pseudomonas aeruginosa; oxidase-positive, blue-green pigment (pyocyanin) & grape-like odor.

• Hosts: Burn patients, CF lungs (biofilm alginate), ventilator-associated pneumonia, catheter UTI, hot-tub folliculitis.

• Virulence: Exotoxin A (ADP-ribosylates EF-2), elastase, quorum sensing.

• Therapy

  • Anti-pseudomonal $\beta$-lactam (piperacillin-tazo, cefepime, ceftazidime, meropenem) $\pm$ aminoglycoside or fluoroquinolone.

  • MDR strains: ceftolozane-tazo, ceftazidime-avibactam, colistin.

• Infection-Control: Drain disinfection, surveillance cultures, antibiotic stewardship.

Q Fever

• Agent: Coxiella burnetii, obligate intracellular, spore-like small cell variant; Category B bioterror threat.

• Reservoir: Domestic ruminants; shed in birth products, urine, feces.

• Transmission: Inhalation of contaminated barn dust; infectious dose <10 organisms.

• Clinical

  • Acute: high fever, severe headache, atypical pneumonia, hepatitis (↑ ALT > AST).

  • Chronic: culture-negative endocarditis (valvular defects).

• Diagnosis: Phase II IgM/IgG by IFA (acute); Phase I IgG ≥$1:800$ indicates chronic.

• Treatment: Doxycycline 100 mg bid × 14 d (acute); doxy + hydroxychloroquine (alkalinizes phagolysosome) × 18–24 mo (chronic).

• Vaccine: Q-Vax (whole-cell) in Australia; pre-screen for antibodies to avoid hypersensitivity.

Rat-Bite Fever

• Microbes

  • North America: Streptobacillus moniliformis (pleomorphic Gram-neg).

  • Asia: Spirillum minus (Gram-neg spirochete).

• Transmission: Bite/scratch of rats, handling feeder rodents, contaminated unpasteurized “rat-milk” drinks in history.

• Clinical: Fever, migratory polyarthralgia, maculopapular rash on palms/soles; untreated mortality $\approx 10\%$.

• Therapy: IV penicillin G × 5–7 d then oral; doxycycline for PCN allergy.

Relapsing Fever

• Agent: Tick-borne (Ornithodoros) or louse-borne Borrelia spp. (e.g., B. hermsii, B. recurrentis).

• Mechanism of Relapse: Antigenic variation of outer membrane proteins causes recurring febrile spikes every $\sim7$ d.

• Clinical: Acute fever ± jaundice, then afebrile, then relapse (up to $10$ cycles in louse-borne).

• Management: Doxycycline; Jarisch-Herxheimer reaction common.

• Prevention: Louse control, avoid rodent-infested cabins.

Salmonellosis (Nontyphoidal)

• Agent: Salmonella enterica serovars Enteritidis, Typhimurium; motile, H2S-producing.

• Source: Undercooked poultry, eggs, reptiles.

• Pathogenesis: Invades M-cells → PMN response limited to GI tract (no capsule).

• Clinical: Watery → sometimes bloody diarrhea, nausea, fever for $3–7$ d.

• Treatment: Supportive oral rehydration; fluoroquinolone or azithro only in severe, infants, elderly, immunocompromised.

• Control: Proper cooking to $\ge 74\,^{\circ}\text{C}$, hand hygiene post-reptile handling.

Scarlet Fever

• Pathogen: GAS producing erythrogenic exotoxin (encoded by lysogenic phage).

• Clinical Triad: 1) sandpaper rash (trunk → extremities sparing palms/soles), 2) circumoral pallor, 3) “strawberry tongue.”

• Pastia’s lines (axillary/inguinal linear erythema).

• Complications: Rheumatic fever (prevented by abx), PSGN (not prevented).

• Treatment: Penicillin V × 10 d or amoxicillin.

Shigellosis

• Agents: S. sonnei (US), S. flexneri, S. dysenteriae type 1 (Shiga toxin → $60\%$ fatal HUS).

• Infective Dose: $\le 10$ organisms—acid-resistant.

• Clinical: High fever, cramps, watery → bloody mucus diarrhea, seizures in children.

• Treatment: Oral rehydration; ciprofloxacin or azithro shortens excretion; avoid anti-motility.

• Public-Health: Day-care outbreaks; hand-washing critical.

Staphylococcal Food Poisoning

• Mechanism: Preformed heat-stable enterotoxin (SEA > SEB); incubation $1–6$ h.

• Presentation: Abrupt vomiting, cramps, sometimes diarrhea; recovery within <24 h.

• Treatment: Supportive (IV fluids, antiemetics).

• Prevention: Keep cooked foods $\le 4^{\circ}\text{C}$ or $\ge 60^{\circ}\text{C}$; hand hygiene for food handlers.

Syphilis

• Stages & Key Findings

  • Primary: painless chancre $\sim3$ wk post-exposure.

  • Secondary: rash incl. palms/soles, condyloma lata, alopecia.

  • Latent: seroreactive, no symptoms.

  • Tertiary: gummas, tabes dorsalis, aortitis.

  • Congenital: snuffles, saber shins, Hutchinson teeth.

• Diagnosis: Nontreponemal (VDRL/RPR) titer → treponemal (FTA-ABS, TP-EIA) confirm.

• Therapy: Benzathine penicillin G 2.4 MU IM (single for early; weekly × 3 for late).

• Public-Health: Screen all pregnant women; rising incidence in MSM.

Tetanus

• Agent: Clostridium tetani spores; exotoxin tetanospasmin blocks $\text{GABA}/\text{glycine}$ release.

• Clinical: Trismus, risus sardonicus, opisthotonos; autonomic instability.

• Management

  • Wound care, metronidazole, TIG $250–500$ IU IM, benzodiazepines, ICU.

  • Active immunization: Td/Tdap booster every $10$ y.

• Prophylaxis Algorithm: If wound dirty + ≥$5$ y since booster → give Tdap; if never immunized → TIG + series.

Trachoma

• Agent: Chlamydia trachomatis serovars A–C; obligate intracellular.

• Transmission: “F-diagram” (fomites, flies, fingers), facial secretions of children.

• WHO SAFE Strategy: Surgery for trichiasis, Antibiotics (azithro mass drug administration), Facial cleanliness, Environmental improvement.

• Burden: Leading infectious cause of blindness (≈$1.9$ million visually impaired).

• Treatment: Single oral azithromycin 20 mg/kg annually × 3 y in endemic villages.

Tuberculosis

• Pathogen: M. tuberculosis complex; acid-fast due to mycolic acids.

• Natural History: Primary infection → latent (LTBI) $\approx$$90\%$; reactivation with immunosuppression.

• Diagnosis: CXR upper-lobe cavitary lesions, IGRA/PPD, sputum AFB smear & culture (gold standard) $6–8 wk.

• Treatment: RIPE × 2 mo → RI × 4 mo; directly observed therapy.

• Prevention: BCG (live attenuated M. bovis) reduces severe pediatric TB and leprosy; variable efficacy in adults.

Tularemia

• Agent: Francisella tularensis subspp. tularensis (Type A) & holarctica (Type B).

• Routes: Ulceroglandular (tick/deer fly bite), pneumonic (aerosolized), oculoglandular, typhoidal.

• Clinical: Painful ulcer + regional lymphadenitis; pneumonia if inhaled.

• Therapy: Streptomycin 1 g IM bid 7–10 d; gentamicin IV alternative.

• Lab Safety: BSL-3; potential bioterror agent.

Typhoid Fever

• Organism: Salmonella Typhi (Vi capsule for intracellular survival).

• Pathogenesis: Invades Peyer’s patches → bacteremia; stealth via Vi capsule; gallbladder carriage ("Typhoid Mary").

• Clinical: Stepwise fever rise, relative bradycardia (Faget sign), abdominal pain, rose spots, hepatosplenomegaly; intestinal hemorrhage/perforation week 3.

• Diagnosis: Blood culture week 1, stool/urine week ≥3; bone marrow most sensitive.

• Treatment: Ceftriaxone or azithromycin (MDR); emerging XDR requires carbapenems.

• Vaccines: Oral live attenuated Ty21a (4 caps), intramuscular Vi polysaccharide (every 2 y).

Vibrio vulnificus Infection

• Ecology: Halophilic Gram-neg curved rod; warm seawater (Gulf Coast).

• Risk Factors: Chronic liver disease, hemochromatosis (iron stimulates growth).

• Clinical: Primary sepsis after raw oysters → shock, hemorrhagic bullae; necrotizing wound infection within <24 h of seawater exposure.

• Management: IV doxycycline + 3rd-gen cephalosporin; aggressive debridement; mortality >50\% in septic cases.

• Prevention: Avoid raw shellfish in liver disease; cover wounds near saltwater.

Yersiniosis

• Agent: Yersinia enterocolitica (pseudoappendicitis), rarely Y. pseudotuberculosis.

• Transmission: Undercooked pork (chitterlings), unpasteurized milk, contaminated water; can multiply at $4^{\circ}\text{C}$ (fridge).

• Clinical: Fever, RLQ pain (mesenteric adenitis), diarrhea (may be bloody), reactive arthritis/erythema nodosum post-infection.

• Diagnosis: Stool culture on CIN agar (“bull’s-eye” colonies).

• Treatment: Usually supportive; severe or immunocompromised: fluoroquinolones or TMP-SMX.

• Public-Health: Mimics appendicitis → unnecessary surgeries; clinician awareness reduces cost.