---

🏥 HARRISON'S CHAPTERS 131-146: Community Acquired Infections

Complete Study Guide for Future Doctors

---

CHAPTER 131 - PNEUMONIA

Definition

• Community-acquired (CAP) - acquired outside hospital
• Hospital-acquired (HAP) - acquired ≥48h after admission
• Ventilator-associated (VAP) - acquired ≥48h after intubation
• Healthcare-associated pneumonia (HCAP) - this category has been discontinued because it did not reliably predict MDR organisms

Aspiration pneumonia accounts for 5-15% of CAP cases. It is a continuum with CAP and HAP, not a separate category.

---

Pathophysiology

1. Inflammatory trigger (often viral + microaspiration)
2. Cytokine/chemokine release → selective bacterial growth (e.g., S. pneumoniae, P. aeruginosa)
3. Positive feedback loop - inflammation → more bacterial growth → more inflammation

• Microaspiration of oropharyngeal flora (most common)
• Inhalation
• Hematogenous spread
• Direct mucosal dispersion

• Nasal hairs/turbinates
• Branching airway anatomy
• Mucociliary clearance
• Gag/cough reflex
• Alveolar macrophages
• Neutrophil recruitment

---

Etiology of CAP

• Mycoplasma pneumoniae
• Chlamydia pneumoniae
• Legionella pneumophila

---

Antimicrobial Resistance - Key Facts

• Penicillin-resistant S. pneumoniae - very common; mechanism = altered penicillin-binding proteins
• Macrolide-resistant M. pneumoniae - Japan 30%, China 95%, US/France 5-13% (mutation in 23S rRNA domain V)
• ESBL-producing gram-negatives - treat with carbapenems
• Fluoroquinolone-resistant E. coli - increasing in community

---

CAP Treatment Algorithm

Outpatient CAP Treatment

• Amoxicillin 1g TID OR
• Doxycycline 100mg BD OR
• Azithromycin (in areas with low pneumococcal macrolide resistance)

• Amoxicillin-clavulanate + macrolide or doxycycline OR
• Respiratory fluoroquinolone (levofloxacin/moxifloxacin) alone

Inpatient CAP Treatment (Non-ICU)

• Beta-lactam + macrolide OR
• Respiratory fluoroquinolone alone

ICU-level CAP

• Beta-lactam + macrolide OR beta-lactam + fluoroquinolone
• Add vancomycin/linezolid if MRSA risk present
• Add anti-pseudomonal beta-lactam if P. aeruginosa risk present

Aspiration Pneumonia

• Routine anaerobic coverage is NOT needed unless: poor dentition, lung abscess, or necrotizing pneumonia

---

Severity Assessment

• C - Confusion (new)
• U - Urea >7 mmol/L (BUN >20 mg/dL)
• R - Respiratory rate ≥30/min
• B - Blood pressure <90 systolic or ≤60 diastolic
• 65 - Age ≥65 years

---

HAP and VAP

• Most important pathogens: P. aeruginosa, Staphylococcus aureus, enteric gram-negatives (Klebsiella, Enterobacter, E. coli, Acinetobacter)
• Key principle: Empirical therapy must cover MDR gram-negatives and MRSA
• De-escalate based on culture results and clinical improvement

---

Prevention

• Pneumococcal vaccines: PCV15 or PCV20 (preferred in adults ≥65)
• Influenza vaccine: Annual, reduces secondary bacterial pneumonia
• Smoking cessation
• Elevate head of bed 30-45° to prevent VAP
• Chlorhexidine oral decontamination in ventilated patients

---

CHAPTER 132 - LUNG ABSCESS (Legionella Pneumonia)

Legionella Pneumonia

• Older male, smoker, immunocompromised
• Relative bradycardia (pulse-temperature dissociation)
• Hyponatremia
• Elevated LFTs
• Diarrhea (watery)
• Confusion/encephalopathy
• Failure to respond to beta-lactams

• Urinary antigen - best for serogroup 1 (sensitivity 70-90%, rapid)
• BAL culture on BCYE agar (buffered charcoal yeast extract) - gold standard
• PCR

• Azithromycin (preferred) - 500mg daily x 7-10 days
• Levofloxacin 750mg daily x 5-7 days (good penetration)
• NOT beta-lactams (no cell wall; Legionella is intracellular)

---

CHAPTER 133 - INFECTIVE ENDOCARDITIS (IE)

Definition

Modified Duke Criteria

1. Positive blood cultures with typical IE organisms (≥2 separate cultures of S. aureus, viridans streptococci, S. gallolyticus, HACEK organisms, Enterococcus faecalis without primary focus)
2. Echocardiographic evidence (vegetation, abscess, new dehiscence of valve prosthesis)
3. New valvular regurgitation

1. Predisposing condition (valve disease, IV drug use)
2. Fever ≥38°C
3. Vascular phenomena (septic emboli, Janeway lesions, intracranial hemorrhage)
4. Immunologic phenomena (Osler nodes, Roth spots, RF positive, glomerulonephritis)
5. Positive blood cultures not meeting major criteria

---

Etiology

---

Clinical Features

• Fever (90%)
• New/changing murmur (80%)
• Splenomegaly (subacute)
• Septic emboli → stroke, pulmonary emboli (right-sided IE)

---

Right-sided vs Left-sided IE

---

Treatment

• Penicillin G 12-18 MU/day x 4 weeks (or ceftriaxone 2g daily x 4 weeks)

• MSSA: Nafcillin/oxacillin 12g/day x 6 weeks
• MRSA: Vancomycin 15-20 mg/kg q8-12h x 6 weeks
• Gentamicin is NOT recommended for native valve S. aureus IE (nephrotoxic, no benefit)

• Longer courses (6 weeks)
• Often requires surgical replacement

• Heart failure due to valve destruction
• Paravalvular abscess
• Resistant organisms (fungal, highly resistant)
• Large vegetations with recurrent emboli
• Failure to sterilize after 7-10 days appropriate antibiotics

---

CHAPTER 134 - INFECTIONS OF SKIN, MUSCLE, AND SOFT TISSUES

Anatomic Approach to Soft Tissue Infections

---

Key Skin Infections Summary

Impetigo

• Layer: Epidermis
• Organisms: S. aureus (bullous) and S. pyogenes (non-bullous)
• Features: Honey-colored crusted lesions (non-bullous), fragile bullae (bullous)
• Tx: Mupirocin topical (mild); oral dicloxacillin/cephalexin (extensive)

Erysipelas

• Layer: Superficial dermis and lymphatics
• Organism: Group A Streptococcus (S. pyogenes)
• Features: Sharply demarcated, raised, fiery red, painful plaque; peau d'orange skin; rapid lateral spread; often face or leg; systemic toxicity
• Tx: Penicillin V or amoxicillin (oral); IV penicillin G if severe

Cellulitis

• Layer: Deep dermis and subcutaneous fat
• Organisms: S. aureus (including MRSA), Group A/B/C Streptococci
• Features: Erythema, warmth, swelling, pain - NOT sharply demarcated (vs. erysipelas)
• Tx: Cephalexin or dicloxacillin (mild); IV cefazolin (moderate); vancomycin if MRSA suspected

Folliculitis, Furuncles, Carbuncles

• Layer: Hair follicle
• Organisms: S. aureus (most), Pseudomonas (hot-tub folliculitis in chlorinated water)
• Furuncle = deep abscess of single follicle
• Carbuncle = multiple interconnected furuncles
• Tx: I&D (drainage) is primary; add antibiotics if systemic toxicity or MRSA

Necrotizing Fasciitis (NF) - THE SURGICAL EMERGENCY

• Pain out of proportion to skin findings
• Bullae
• Crepitus (gas in tissues - best felt/heard on palpation)
• Skin necrosis, dusky discoloration
• Rapid progression, systemic toxicity
• CT scan shows fascial thickening and gas in planes

Fournier's gangrene = NF of perineum/genitalia - urologic emergency

Myositis / Gas Gangrene (Clostridial Myonecrosis)

• Layer: Muscle
• Organisms: Clostridium perfringens (most important)
• Features: Severe pain, bronze/gray skin, crepitus, gas on X-ray, sweet/dishwater foul odor, rapid progression
• Treatment: Immediate surgery (amputation often needed) + high-dose penicillin G + clindamycin (blocks toxin production)

---

MRSA Skin Infections

• TMP-SMX DS (trimethoprim-sulfamethoxazole) - first line outpatient
• Doxycycline
• Clindamycin (check D-zone test for inducible resistance)
• Vancomycin (IV, severe)

---

CHAPTER 135 - INFECTIOUS ARTHRITIS

Overview

---

Pattern of Joint Involvement

---

Route of Infection

1. Hematogenous (most common) - bacteria escape from synovial capillaries (no limiting basement membrane) within hours
2. Contiguous spread from adjacent bone/soft tissue
3. Direct inoculation (surgery, injection, bite, trauma)

---

Synovial Fluid Analysis - CRITICAL TABLE

---

Gonococcal Arthritis

1. DGI (Disseminated Gonococcal Infection): Bacteremic phase - migratory polyarthralgia, tenosynovitis, skin lesions (pustules/vesicles), no joint effusion
2. Purulent arthritis phase: Localized to 1-2 joints, no skin lesions

---

Prosthetic Joint Infections (PJI)

• Early (<3 months post-op): S. aureus, gram-negatives (intraoperative contamination)
• Delayed (3-24 months): Coagulase-negative staphylococci, Cutibacterium acnes
• Late (>24 months): Hematogenous seeding

---

CHAPTER 136 - OSTEOMYELITIS

Overview

Hematogenous Osteomyelitis

• Organisms: S. aureus most common; gram-negatives (IV drug users, urinary source)
• Features: Back pain, fever, localized tenderness over spine
• Diagnosis: MRI (gold standard - shows marrow edema); bone biopsy for culture
• Complication: Epidural abscess → cord compression → EMERGENCY

Chronic Osteomyelitis

• Often follows hematogenous acute osteomyelitis or traumatic/surgical infection
• Sequestrum = dead bone (nidus for infection, must be removed)
• Involucrum = new periosteal bone formation around sequestrum
• Cloaca = opening from sequestrum to skin surface (draining sinus)
• Treatment: Surgical debridement + prolonged antibiotics (4-6 weeks)

Diabetic Foot Osteomyelitis

• Contiguous spread from infected foot ulcer
• Polymicrobial (S. aureus + gram-negatives + anaerobes)
• "Probe-to-bone test" - if metal probe hits bone through ulcer = high likelihood of osteomyelitis
• MRI superior to plain X-ray (changes take 10-21 days to appear on X-ray)

---

CHAPTER 137 - ABDOMINAL INFECTIONS AND ABSCESSES

Peritonitis

• Occurs in cirrhosis with ascites
• Organism: Monomicrobial (E. coli, Klebsiella, Streptococcus pneumoniae)
• Diagnosis: Ascitic fluid PMN count ≥250/mm³
• Treatment: Cefotaxime 2g IV q8h x 5 days
• Prophylaxis: Norfloxacin/TMP-SMX (long-term in prior SBP) or ceftriaxone (acute GI bleed)

• Polymicrobial (E. coli, Bacteroides fragilis, Enterococcus, Clostridium)
• Treatment: Surgical source control + broad-spectrum antibiotics

Intraabdominal Abscesses

• Usually result from ruptured appendix, diverticulitis, Crohn's, post-surgical
• Polymicrobial (enteric gram-negatives + anaerobes)
• Treatment: Drainage (percutaneous or surgical) + antibiotics
• Drainage is essential - antibiotics alone rarely suffice

Liver Abscess

• Source: Portal vein bacteremia from bowel, biliary tract infection
• Organisms: E. coli, Klebsiella, Streptococcus milleri group, anaerobes
• CT shows hypodense lesion(s)
• Treatment: Percutaneous drainage + 4-6 weeks antibiotics

• Young adult male from endemic area
• "Anchovy sauce" pus (chocolate-colored)
• Right lobe, solitary
• Serology positive (>95% sensitive)
• Treatment: Metronidazole 750mg TID x 10 days (no drainage usually needed)

---

CHAPTER 138 - ACUTE INFECTIOUS DIARRHEAL DISEASES AND BACTERIAL FOOD POISONING

Pathogenic Mechanisms

1. Inoculum Size

• Shigella, E. coli O157, Giardia, Entamoeba: 10-100 organisms
• Vibrio cholerae: 10⁵-10⁸ organisms needed This explains why Shigella/E. coli O157 spreads person-to-person while Salmonella needs to grow in food first.

2. Adherence

• V. cholerae adheres via toxin-coregulated pilus (TCP) and colonization factors
• EPEC and EHEC cause "attaching and effacing" lesions - destroy brush border microvilli

3. Toxin Production

4. Invasion

• Shigella, Salmonella, EIEC, Yersinia invade colonic mucosa
• Campylobacter invades small bowel

---

Clinical Syndromes

---

Bacterial Food Poisoning - Key Organisms

Memory trick for short incubation (preformed toxins): "STAPH doesn't need time" - 1-6 hours = S. aureus or B. cereus.

---

Treatment Approach

• Shigella (ciprofloxacin or azithromycin)
• Cholera (doxycycline - reduces duration)
• Typhoid (fluoroquinolone or ceftriaxone)
• Traveler's diarrhea (if severe: ciprofloxacin or azithromycin)
• C. difficile (see Ch. 139)

---

CHAPTER 139 - CLOSTRIDIOIDES DIFFICILE INFECTION (CDI), INCLUDING PSEUDOMEMBRANOUS COLITIS

Definition

---

Etiology and Epidemiology

• Organism: Obligately anaerobic, gram-positive, spore-forming bacillus
• Spores survive in environment for months and resist alcohol-based hand sanitizers
• Only hand-washing with soap and water removes spores effectively!

1. Clindamycin (historically first identified)
2. Ampicillin/amoxicillin
3. Second/third generation cephalosporins (cefotaxime, ceftriaxone, ceftazidime)
4. Fluoroquinolones (most recent class - associated with hypervirulent NAP1/BI/027 strain)

• ~462,100 CDI cases in US in 2017
• Fecal colonization rate: Community 1-3% vs. Hospital >20% after >2 weeks
• Neonates: Frequently colonized but disease is extremely rare

• Antibiotic use
• Older age
• Hospitalization / nursing home
• GI surgery
• Enteral tube feeding
• PPI use (modest risk)
• Prior CDI

---

Pathogenesis

---

Clinical Spectrum

---

Diagnosis

• NAAT (PCR): Very sensitive but does not distinguish colonization from infection - use with clinical judgment
• EIA for toxin A and B: Less sensitive but more specific for active disease
• Two-step algorithm: NAAT screen → EIA toxin confirmation (recommended in guidelines)
• Colonoscopy/sigmoidoscopy: Shows pseudomembranes; rarely needed but diagnostic

---

Treatment

Metronidazole is no longer first-line (inferior cure rates vs vancomycin). Use only in fulminant CDI as IV add-on.

---

CHAPTER 140 - URINARY TRACT INFECTIONS: CYSTITIS, PROSTATITIS AND PYELONEPHRITIS

Pathogens

---

Pathogenesis

1. Flushing action of urination - primary host defense
2. Obstructive uropathy - congenital anomalies, BPH, stones
3. Sexual intercourse - risk increases with frequency (4.8x with 5x/week)
4. Urinary instrumentation - catheters, nephrostomy tubes
5. Disruption of urogenital microbiome - diaphragm/spermicide use, menopause
6. Diabetes mellitus - 2-3x higher risk (impaired cytokine secretion, bladder dysfunction)
7. SGLT-2 inhibitors - FDA safety warning for increased UTI risk (glucosuria)

---

Clinical Presentations

Acute Uncomplicated Cystitis

• Dysuria, frequency, urgency, suprapubic pain
• No fever (fever = suggests upper tract)
• Positive dipstick: leukocyte esterase and/or nitrites
• Urine culture: ≥10³ CFU/mL (significant in symptomatic women)

• Nitrofurantoin 100mg ER BD x 5 days (first line; not for pyelonephritis - poor renal tissue levels)
• TMP-SMX DS BD x 3 days (first line if local resistance <20%)
• Fosfomycin 3g single dose
• Fluoroquinolones - effective but reserve (high resistance, C. diff risk)

Acute Pyelonephritis

• Flank pain/tenderness (CVA tenderness)
• High fever, rigors
• Nausea/vomiting
• Bacteriuria + pyuria + casts in urine
• Blood cultures positive in ~20%

• Outpatient (uncomplicated): Ciprofloxacin 500mg BD x 7 days (if susceptibility known) or TMP-SMX x 14 days
• Inpatient: IV ceftriaxone or fluoroquinolone; switch to oral when improving
• Complicated / ESBL: Ertapenem or meropenem

Prostatitis

---

CHAPTER 141 - SEXUALLY TRANSMITTED INFECTIONS: OVERVIEW AND CLINICAL APPROACH

Overview

---

Major STIs Summary Table

---

Syphilis - Detailed

• Early: Aseptic meningitis, cranial nerve palsies, iritis
• Late: Tabes dorsalis (posterior column degeneration), general paresis

• Screening: RPR or VDRL (non-treponemal; titers reflect disease activity; false positives common)
• Confirmation: FTA-ABS or TPHA (treponemal; remains positive lifelong)

• Primary/Secondary/Early latent: Benzathine penicillin G 2.4 MU IM single dose
• Late latent/unknown duration: 2.4 MU IM weekly x 3 doses
• Neurosyphilis: Aqueous penicillin G 18-24 MU/day IV x 10-14 days
• Penicillin allergy: Doxycycline 100mg BD x 14-28 days (not in pregnancy)

---

Pelvic Inflammatory Disease (PID)

• Ascending infection from lower genital tract
• Organisms: N. gonorrhoeae, C. trachomatis, anaerobes, M. hominis
• Clinical: Lower abdominal pain, cervical motion tenderness (Chandelier sign), adnexal tenderness, fever
• Complications: Tubo-ovarian abscess (TOA), chronic pelvic pain, ectopic pregnancy, infertility

• Outpatient: Ceftriaxone 500mg IM + doxycycline 100mg BD x 14 days ± metronidazole 500mg BD
• Inpatient (if severe/TOA): IV cefoxitin + doxycycline, or clindamycin + gentamicin

---

Chapters 142 - STIs Continued (specific organisms - see Ch 141 table)

---

CHAPTER 143 - ACUTE MENINGITIS

Definition

---

Common Causative Organisms by Age

---

Classic Clinical Triad (only 44% of patients have all three)

1. Fever
2. Neck stiffness (nuchal rigidity)
3. Altered mental status

• Kernig's sign: Inability to extend knee when hip is flexed at 90°
• Brudzinski's sign: Passive neck flexion causes involuntary knee/hip flexion
• Photophobia, phonophobia
• Petechial/purpuric rash = meningococcal meningitis until proven otherwise!

---

Lumbar Puncture (LP) - CSF Analysis

Contraindications to LP before CT: Papilledema, focal neurological signs, new seizures, immunocompromise, altered consciousness. If any present → CT first, then LP (but DO NOT delay antibiotics for CT!)

---

Empirical Treatment - The 60-Minute Goal

MEDICAL EMERGENCY: Start antibiotics within 60 minutes of patient arrival.

• Ceftriaxone 2g IV q12h (covers S. pneumoniae, N. meningitidis, H. influenzae)
• + Vancomycin (covers cephalosporin-resistant S. pneumoniae)
• + Dexamethasone 0.15 mg/kg q6h x 4 days (give BEFORE or WITH first dose of antibiotic - reduces hearing loss and neurological sequelae in pneumococcal meningitis)
• + Acyclovir (covers HSV encephalitis in differential)
• + Doxycycline (during tick season - covers tick-borne organisms)

• Age <3 months, age >55, pregnancy, immunocompromised
• Listeria is intrinsically resistant to cephalosporins!

• Patients with otitis, sinusitis, mastoiditis (risk of anaerobic infection)

• Vancomycin + Meropenem (covers P. aeruginosa and resistant gram-negatives)

---

Pathogen-Specific Therapy (When Culture Known)

---

Complications

• Cerebral edema
• Cerebral herniation
• Hydrocephalus
• Vasculitis → stroke
• Subdural empyema
• Septic venous sinus thrombosis
• SIADH → hyponatremia
• Hearing loss (most common long-term complication, especially pneumococcal)
• Waterhouse-Friderichsen syndrome (meningococcal) = bilateral adrenal hemorrhage → adrenal crisis

---

CHAPTER 144 - CHRONIC AND RECURRENT MENINGITIS

Key Causes

---

Tuberculous Meningitis (TBM) - Key Points

• Most lethal form of TB with high mortality/morbidity if untreated
• Subacute onset: weeks of headache, fever, weight loss → confusion, cranial nerve palsies
• CSF: Lymphocytic pleocytosis (100-400 cells), elevated protein, very low glucose, elevated ADA
• Diagnosis: AFB stain (low sensitivity <25%), culture (gold standard but slow), PCR (moderate sensitivity)
• Basal meningitis → basilar exudates → CN palsies (III, IV, VI especially)
• Hydrocephalus is common

---

Cryptococcal Meningitis

• Classic in AIDS (CD4 <100 cells/μL), also organ transplant recipients
• Subacute/chronic headache, fever, minimal neck stiffness
• India ink stain of CSF - shows encapsulated yeasts (like halo around them)
• CSF cryptococcal antigen (CrAg) - highly sensitive/specific
• Elevated opening pressure - key management challenge (serial LP or lumbar drain)

• Induction: Liposomal amphotericin B + flucytosine x 2 weeks
• Consolidation: Fluconazole 400mg daily x 8 weeks
• Maintenance: Fluconazole 200mg daily (until immune reconstitution in HIV)

---

CHAPTER 145 - BRAIN ABSCESS (Encephalitis)

Brain Abscess

Etiology by Source

---

Clinical Features

• Headache (most common, >70%)
• Fever (only 50% - often absent in brain abscess!)
• Focal neurological deficits
• Seizures
• Signs of raised ICP
• Classic triad of fever, headache, focal deficit in only 20%

---

Diagnosis

• MRI with gadolinium - superior to CT
• Ring-enhancing lesion with surrounding edema
• Single lesion: Suspect bacterial abscess or primary tumor
• Multiple ring-enhancing lesions in AIDS: Toxoplasma vs. CNS lymphoma

---

Treatment of Bacterial Brain Abscess

1. Aspiration/drainage (surgical) - provides diagnosis AND treatment; essential if >2.5cm
2. Antibiotics (6-8 weeks):
   • Empirical: Ceftriaxone + metronidazole + vancomycin
   • Post-neurosurgery/trauma: Vancomycin + cefepime or meropenem
3. Dexamethasone - use only if severe mass effect (may impair antibiotic penetration)
4. Antiepileptic drugs - for seizures

---

CHAPTER 146 - INFECTIOUS COMPLICATIONS OF BITES

Overview

---

Dog Bites

• >4.7 million dog bites/year in US; responsible for 80% of all animal bites
• 15-20% of dog bites become infected
• Children most common victims (boys 5-9 highest risk)
• Infection manifests 8-24h after bite: cellulitis, purulent discharge, pain

• Pasteurella multocida
• β-hemolytic streptococci
• Staphylococcus spp. (including MRSA)
• Neisseria species
• Eikenella corrodens
• Capnocytophaga canimorsus - splenectomized/immunocompromised patients at very high risk of fatal septicemia!
• Anaerobes: Bacteroides, Fusobacterium, Prevotella

---

Cat Bites

• Cat bites become infected in 30-50% (higher rate than dogs because sharper, deeper puncture wounds)
• Most important organism: Pasteurella multocida - very common in cat bite infections; rapid onset (within 12h), intense pain, erythema, swelling
• Also: streptococci, staphylococci, anaerobes, Bartonella henselae (cat-scratch disease)

• Cat scratch or bite (especially from kittens)
• 1-3 week incubation
• Regional lymphadenopathy (tender, may suppurate)
• Low-grade fever, malaise
• Diagnosis: Serology or PCR
• Treatment: Self-limited; azithromycin for lymphadenitis

---

Human Bites

• Highest infection rate of any bite (human oral flora is the most diverse and pathogenic)
• Two types:
  1. Occlusal/true bite - directly inflicted
  2. Fight bite / Clenched-fist injury (CFI) - knuckles striking teeth; highly dangerous; often involves joint space

• Streptococcus spp.
• Staphylococcus aureus
• Eikenella corrodens (characteristic of human bites; not in animal bites)
• Anaerobes: Fusobacterium, Prevotella, Peptostreptococcus

---

Snake Bites

• Venomous vs. non-venomous
• Local infection risk: mouth flora of snake; Gram-negatives (Morganella, Aeromonas, Providencia) + anaerobes
• Treatment of envenomation: antivenom; supportive care
• Antibiotic prophylaxis is NOT routinely recommended for all snake bites

---

General Bite Wound Management

• Assess wound depth, contamination, neurovascular status
• Blood cultures if systemic signs
• Clean thoroughly - copious irrigation with saline (most important step)
• Debride devitalized tissue
• Empirical antibiotics

• All cat bites
• Dog bites: moderate-severe, hands/face/genitals, immunocompromised, asplenic, diabetic, edematous areas
• All human bites

• Penicillin allergy: Doxycycline + metronidazole, OR moxifloxacin

• Generally leave open for infected or high-risk bites
• Face bites: can close primarily (good blood supply, cosmetic importance)

---

🧠 QUICK-RECALL MNEMONICS

CURB-65 (Pneumonia severity)

Duke Criteria for IE - MAJOR = "HERO"

C. diff Risk Antibiotics - "CAFF"

Meningitis Organisms by Age

• Neonates: "Group B Strep, E. coli, Listeria" = "BEL (Beautiful Early Life)"
• Adults: "S. pneumo, N. meningitidis" = the two most common

Syphilis Rash Memory

NF (Necrotizing Fasciitis) Red Flags

Bite Wound Drug

---

📊 MASTER COMPARISON TABLE

---

---

1. Images help anchor the anatomy - the skin layer diagram is especially important for understanding why erysipelas vs cellulitis vs NF differ clinically.
2. Tables let you rapidly compare organisms, CSF findings, and treatments.
3. Mnemonics at the end help on exam day.
4. Focus especially on the emergency conditions: Meningitis (60-min rule), Necrotizing Fasciitis (immediate surgery), and Bacterial Endocarditis (surgical indications).