SIRS and Sepsis Criteria
SIRS: Temp >38° or <36°, HR >90, RR >20, WBC >12k or <4k. Sepsis = SIRS + suspected infection.
Systemic Inflammatory Response Syndrome
The clinical criteria for identifying sepsis — the leading cause of hospital death
SIRS (2+ criteria): fever or hypothermia, tachycardia, tachypnea or low pCO₂, leukocytosis or leukopenia. Sepsis: SIRS + suspected infection source. Severe sepsis: organ dysfunction. Septic shock: sepsis + refractory hypotension (MAP <65) despite IV fluids. Management: blood cultures before antibiotics, broad-spectrum antibiotics within 1 hour, 30 mL/kg IV fluid bolus, vasopressors for refractory hypotension.
► Show full breakdown RR
>20 breaths/min or PaCO₂ <32
WBC
>12,000 or <4,000 or >10% bands
Meningitis Causes by Age
Meningitis by age: Neonates (GBS + E. coli + Listeria), Children (N. meningitidis + S. pneumoniae), Adults (S. pneumoniae #1)
Meningitis Pathogens by Age
The most common bacterial meningitis pathogens shift with age
Neonates (<1 mo): Group B Strep, E. coli, Listeria. Cover with ampicillin + gentamicin. Children (1 mo–18 yr): N. meningitidis, S. pneumoniae, H. influenzae (now rare — vaccine). Adults: S. pneumoniae (most common), N. meningitidis. Elderly/immunocompromised: add Listeria — cover with ampicillin. Classic triad: fever + headache + neck stiffness. +/- Kernig's and Brudzinski's signs.
CSF Analysis
CSF: Bacterial = cloudy, ↑WBC (PMNs), ↑protein, ↓glucose. Viral = clear, ↑lymphs, normal glucose. TB/Fungal = between.
Cerebrospinal Fluid Patterns
CSF patterns that identify the type of meningitis without waiting for culture
Bacterial meningitis: WBC 1000–10000 (neutrophils), protein >100, glucose <45 (or CSF:serum ratio <0.6), cloudy appearance. Viral meningitis: WBC 10–500 (lymphocytes), protein mildly elevated, glucose normal. TB/Fungal: lymphocytic pleocytosis, high protein, low glucose. Opening pressure: bacterial and fungal elevated. India ink for Cryptococcus, acid-fast stain for TB.
Pneumonia Pathogens
CAP: S. pneumoniae #1. Atypical (walking pneumonia): Mycoplasma, Chlamydophila, Legionella. HAP: Gram-negatives + MRSA.
Community vs Hospital-Acquired Pneumonia
Different settings, different organisms — changes the antibiotic choice
CAP typical: S. pneumoniae (lobar), H. influenzae (COPD), Klebsiella (alcoholics). CAP atypical (no sputum, no consolidation, mild): Mycoplasma pneumoniae (young adults), Chlamydophila (gradual), Legionella (water systems, pontiac fever, hyponatremia). HAP/VAP: Pseudomonas, Klebsiella, MRSA. CAP treatment: azithromycin or doxycycline (mild), beta-lactam + macrolide (moderate), antipseudomonal + vancomycin (ICU).
UTI Pathogens
UTI bugs: "EKP SPSS" — E. coli #1 (80%), Klebsiella, Proteus, Staphylococcus saprophyticus (young women), Serratia, Pseudomonas
UTI Causative Organisms
Which bacteria cause UTIs — and the key clinical clues for each
E. coli: 80% of community UTIs — fimbriae (type 1, P fimbriae) mediate adhesion. Staphylococcus saprophyticus: #2 in sexually active young women. Proteus mirabilis: urease → ammonia → struvite (staghorn) kidney stones, alkaline urine. Klebsiella: diabetics, hospital. Enterococcus: requires ampicillin. Diagnosis: pyuria (>5 WBC/hpf), bacteriuria, ± nitrites (Gram-only), leukocyte esterase. Uncomplicated: TMP-SMX or nitrofurantoin 3–7 days.
STI Overview
STIs: Gonorrhea (Gram- diplococci, pus), Syphilis (painless chancre, VDRL+), Chlamydia (intracellular, silent).
Sexually Transmitted Infections
Key features that distinguish the major STIs in clinical presentation
Gonorrhea (N. gonorrhoeae): purulent urethral discharge, Gram- diplococci, joint infections (DGI). Treat: ceftriaxone. Chlamydia (C. trachomatis): most common STI, often asymptomatic, PID, epididymitis, neonatal conjunctivitis. Treat: azithromycin or doxycycline. Syphilis: primary (painless chancre), secondary (maculopapular rash on palms/soles), tertiary (aorta, CNS). Treat: penicillin G. HSV-2: painful vesicles, recurrent.
► Show full breakdown Gonorrhea
Purulent discharge, Gram- diplococci, ceftriaxone
Chlamydia
Silent, intracellular, doxycycline
Syphilis
3 stages, VDRL screen, FTA-ABS confirm
HSV
Painful vesicles, recurrent, acyclovir
Tick-Borne Diseases
Tick diseases: Lyme (bull's-eye rash), Rocky Mountain Spotted Fever (rash starts peripherally), Ehrlichia, Babesia.
Tick-Borne Pathogens
Four major tick-borne infections — different ticks, different presentations
Lyme disease (Borrelia burgdorferi, Ixodes tick): early = erythema migrans (bull's-eye); late = arthritis, heart block, Bell's palsy. Treat: doxycycline. Rocky Mountain Spotted Fever (Rickettsia rickettsii, Dermacentor): fever + rash starts on wrists/ankles → trunk. Treat: doxycycline (do NOT wait for confirmation). Ehrlichiosis: leukopenia, thrombocytopenia, elevated LFTs. Babesia: hemolytic anemia, "Maltese cross" on smear — treat with atovaquone + azithromycin.
Endocarditis
Endocarditis: fever + new murmur. Viridans strep (dental). S. aureus (IV drug use). Duke criteria diagnose.
Infective Endocarditis
Bacterial infection of the heart valves — high morbidity, must not miss
Viridans streptococci: most common IE — after dental procedures, normal valves. S. aureus: aggressive, IV drug users (right-sided, tricuspid), prosthetic valves. S. bovis (now Streptococcus gallolyticus): associated with colon cancer. HACEK organisms: slow-growing Gram- rods. Classic signs: Roth spots, Janeway lesions, Osler nodes, splinter hemorrhages. Blood cultures ×3 before antibiotics. Treat empirically: vancomycin + ceftriaxone.
Foodborne Illness
Food poisoning timing: Staph toxin = 1–6 hr (preformed). Salmonella = 12–48 hr. C. perfringens = 8–16 hr.
Foodborne Illness Incubation Times
The time from eating to symptoms tells you which organism is responsible
Short incubation (preformed toxin, no growth needed): Staph aureus (1–6 hr, mayo/potato salad, vomiting), B. cereus fried rice (1–6 hr emetic) or 8–16 hr diarrheal. Medium (8–16 hr): Clostridium perfringens (reheated meat, gravy, diarrhea no vomiting). Long (12–72 hr): Salmonella (eggs, poultry, reptiles, bloody diarrhea), E. coli O157:H7 (undercooked beef → HUS), Campylobacter (poultry — bloody diarrhea, Guillain-Barré post-infection).
► Show full breakdown 1–6 hr
Staph aureus or B. cereus — preformed toxin
8–16 hr
C. perfringens — toxin made in gut
12–72 hr
Salmonella, Campylobacter — bacterial growth
TORCH Infections
TORCH: Toxoplasma, Other (syphilis/Zika/parvovirus), Rubella, CMV, Herpes/HIV. Congenital infections.
Congenital Infections
Infections that cross the placenta and cause congenital defects
Toxoplasma: cat exposure, chorioretinitis, hydrocephalus, intracranial calcifications. Rubella: "blueberry muffin" rash, cataracts, deafness, heart defects (PDA). CMV: most common congenital infection — periventricular calcifications, sensorineural hearing loss, jaundice. Herpes: skin/eye/mouth or disseminated; C-section if active lesions. Syphilis: saddle nose, Hutchinson's teeth, interstitial keratitis. Zika: microcephaly. Parvovirus B19: hydrops fetalis.
Clostridium difficile
C. diff: antibiotic-associated diarrhea. Spores survive surfaces. Watery diarrhea + leukocytosis after antibiotics = suspect C. diff.
Clostridioides difficile
The most common hospital-acquired infection — caused by treating other infections
Antibiotics disrupt normal flora → C. diff overgrows → toxin A (enterotoxin) + toxin B (cytotoxin) → pseudomembranous colitis. Risk: clindamycin, fluoroquinolones, cephalosporins (3rd gen). Diagnosis: stool toxin PCR. Mild-moderate: oral vancomycin or fidaxomicin (metronidazole second-line). Severe/fulminant: IV metronidazole + oral vancomycin ± colectomy. Recurrence: fecal microbiota transplant (FMT). Contact precautions; alcohol does NOT kill spores — soap and water.
Tuberculosis
TB: acid-fast bacillus. Latent: PPD+ but no symptoms. Active: cough >3 weeks + night sweats + weight loss. RIPE therapy.
Mycobacterium tuberculosis
TB diagnosis, latent vs active disease, and the RIPE treatment regimen
M. tuberculosis: obligate aerobe, slow-growing (24 hr generation time), upper lung lobes (high O₂). Transmission: airborne (droplet nuclei). Latent TB: PPD >10 mm or IGRA+, no symptoms, non-infectious — treat with isoniazid 9 months. Active TB: RIPE for 2 months, then RI for 4 months — Rifampin, Isoniazid, Pyrazinamide, Ethambutol. MDR-TB: resistant to INH and RIF. Ghon complex: primary + lymph node calcification.
► Show full breakdown R
Rifampin — induces P450, red-orange urine
I
Isoniazid — peripheral neuropathy, give B6
P
Pyrazinamide — hyperuricemia, hepatotoxic
E
Ethambutol — optic neuritis, color vision