Anti Microbials – Pharmacology · FCPS Part 1

Master Table (Very detailed, no need to memorize all)

Organism	Important diseases	First-line / preferred drug	Important coverage / use	🚨 Major adverse effect	🎯 MCQ clue
S. aureus – MSSA	Skin abscess, cellulitis, osteomyelitis, septic arthritis, endocarditis, pneumonia	Nafcillin / oxacillin / cefazolin	MSSA	β-lactam allergy	Coagulase +
S. aureus – MRSA	Skin/soft tissue infection, pneumonia, bacteremia, endocarditis	Vancomycin	Serious resistant Gram+	Nephrotoxicity, red-man syndrome	mecA → altered PBP
S. epidermidis	Prosthetic valve/device infection, catheter infection	Vancomycin if resistant	Resistant Gram+; biofilm infections	Vancomycin → nephrotoxicity/red man	Prosthetic device + biofilm
S. saprophyticus	Uncomplicated UTI	Nitrofurantoin	Urinary pathogen	Pulmonary/hepatic toxicity with prolonged use	Young woman + UTI + novobiocin resistant
S. pyogenes (GAS)	Pharyngitis, scarlet fever, impetigo, cellulitis, necrotizing fasciitis	Penicillin	GAS infections	Allergy	β-hemolytic + bacitracin sensitive
S. pneumoniae	Pneumonia, meningitis, otitis media, sinusitis	Ceftriaxone ± vancomycin for serious disease	Serious pneumococcal infection	Ceftriaxone: biliary sludging; vancomycin: nephrotoxicity	Lancet-shaped diplococci + capsule
Enterococcus	UTI, endocarditis, intra-abdominal/biliary infection	Ampicillin if susceptible	Gram+; some endocarditis regimens use synergistic aminoglycoside	Aminoglycosides → nephro/oto	GI/GU flora; intrinsically resistant to many cephalosporins
VRE	UTI, bacteremia, endocarditis	Linezolid / daptomycin	Resistant Gram+	Linezolid → thrombocytopenia; daptomycin → myopathy	Vancomycin-resistant Enterococcus
Listeria monocytogenes	Neonatal meningitis, meningitis in elderly/immunocompromised, pregnancy infection	Ampicillin	Intracellular Gram+ rod	Ampicillin → allergy	Pregnancy/neonate/elderly + meningitis; tumbling motility
Actinomyces	Cervicofacial infection, abscesses	Penicillin	Anaerobic Gram+ branching rod	Penicillin allergy	Sulfur granules + dental/oral infection
Nocardia	Pulmonary infection, brain abscess	TMP-SMX	Immunocompromised patients	Hyperkalemia, marrow suppression, SJS	Weakly acid-fast branching filamentous organism
N. gonorrhoeae	Gonorrhea, PID, epididymitis, septic arthritis, neonatal conjunctivitis	Ceftriaxone	Gonococcal infection	Allergy	Gram− diplococci inside neutrophils; glucose only
N. meningitidis	Meningitis, meningococcemia	Ceftriaxone	Meningococcal disease	Allergy	Petechial/purpuric rash + meningitis
E. coli	UTI, pyelonephritis, neonatal meningitis, diarrhea	Nitrofurantoin for uncomplicated cystitis	Commonest uncomplicated UTI pathogen	Nitrofurantoin → pulmonary/hepatic toxicity	Lactose fermenter + pink colonies on MacConkey
EHEC	Bloody diarrhea, HUS	Supportive	Avoid routine antibiotics	Antibiotics may increase HUS risk	Undercooked beef + bloody diarrhea + HUS
ETEC	Traveler's diarrhea	Usually supportive; antibiotics in selected cases	Watery diarrhea	Depends on agent	Traveler + watery diarrhea
EPEC	Infantile diarrhea	Supportive	Pediatric diarrhea	—	Infant + watery diarrhea
Klebsiella pneumoniae	Lobar pneumonia, UTI, healthcare-associated infection	Depends on susceptibility; carbapenem for ESBL	Resistant Gram− infections	Carbapenems: seizures risk	Currant-jelly sputum; alcoholic patient
Proteus	UTI, pyelonephritis, stones	TMP-SMX / FQ / cephalosporin depending on susceptibility	UTI	Drug-specific	Urease + → struvite stones
Salmonella Typhi	Typhoid fever, bacteremia	Ceftriaxone or susceptible FQ	Systemic Salmonella infection	Ceftriaxone: biliary effects	Rose spots + relative bradycardia
Nontyphoidal Salmonella	Gastroenteritis, bacteremia, osteomyelitis	Usually supportive	GI infection	Antibiotic-related adverse effects	Reptiles/poultry/eggs; sickle-cell osteomyelitis
Shigella	Bacillary dysentery	Azithromycin / ceftriaxone / FQ depending on susceptibility	Invasive diarrhea	Drug-specific	Bloody diarrhea + tenesmus + very low infectious dose
Yersinia enterocolitica	Diarrhea, terminal ileitis, pseudoappendicitis	TMP-SMX / FQ when indicated	GI infection	Drug-specific	Undercooked pork + pseudoappendicitis
Campylobacter jejuni	Bloody diarrhea, enteritis	Azithromycin if treatment indicated	Bacterial gastroenteritis	QT prolongation	Poultry + curved “gull-wing” rods + GBS
Pseudomonas aeruginosa	Pneumonia, burn infection, UTI, otitis externa, keratitis, bacteremia	Pip-tazo / cefepime / ceftazidime / FQ / meropenem	Antipseudomonal coverage	Aminoglycosides → nephro/oto; FQ → tendon toxicity	Blue-green pigment + grape odor
Vibrio cholerae	Cholera	Aggressive fluid replacement ± doxycycline	Secretory diarrhea	Doxycycline: GI/photosensitivity	Rice-water stool
H. pylori	Chronic gastritis, peptic ulcer disease, gastric adenocarcinoma, MALT lymphoma	Combination therapy + PPI	Gastric H. pylori eradication	Depends on regimen	Urease +; curved rod
Legionella pneumophila	Atypical pneumonia	Azithromycin / levofloxacin	Intracellular respiratory pathogen	QT prolongation	Pneumonia + diarrhea + hyponatremia + water exposure
H. influenzae	Meningitis, epiglottitis, pneumonia, otitis	Ceftriaxone for invasive disease	Invasive H. influenzae	Allergy/GI effects	Epiglottitis + chocolate agar
Bordetella pertussis	Whooping cough	Azithromycin	Reduces transmission	QT prolongation	Paroxysmal cough + inspiratory whoop + posttussive vomiting
Brucella	Undulating fever, osteoarticular disease	Doxycycline + rifampin	Intracellular organism	Doxy: photosensitivity; rifampin: hepatotoxicity/CYP induction	Unpasteurized dairy + livestock
Francisella tularensis	Tularemia, ulceroglandular disease, pneumonia	Gentamicin / streptomycin	Severe tularemia	Aminoglycoside → nephro/oto	Rabbit/tick + ulcer + lymphadenopathy
Pasteurella multocida	Cellulitis after animal bite	Amoxicillin-clavulanate	Animal-bite infections	GI upset, allergy	Cat/dog bite
Bartonella henselae	Cat-scratch disease, bacillary angiomatosis	Azithromycin when treatment indicated	Cat-associated infection	QT prolongation	Cat exposure + regional lymphadenopathy
C. trachomatis	Urethritis, cervicitis, PID, epididymitis, neonatal conjunctivitis	Doxycycline; azithromycin in pregnancy	Intracellular STI pathogen	Doxy → photosensitivity; tooth discoloration in developing teeth	STI + intracellular + no typical cell wall
C. psittaci	Atypical pneumonia	Doxycycline	Bird-associated infection	Photosensitivity	Parrot/bird exposure
Mycoplasma pneumoniae	Walking/atypical pneumonia	Azithromycin / doxycycline	Atypical pneumonia	Macrolide → QT; doxy → photosensitivity	No cell wall + cold agglutinins
R. rickettsii	Rocky Mountain spotted fever	Doxycycline	Rickettsial infection	Photosensitivity	Tick + wrists/ankles → trunk/palms/soles
Ehrlichia	Ehrlichiosis	Doxycycline	Tick-borne infection	Photosensitivity	Monocytes + morulae
Anaplasma	Anaplasmosis	Doxycycline	Tick-borne infection	Photosensitivity	Granulocytes + morulae
Coxiella burnetii	Q fever, pneumonia, hepatitis, culture-negative endocarditis	Doxycycline	Intracellular organism	Photosensitivity	Cattle/sheep + no arthropod vector
Treponema pallidum	Syphilis: primary, secondary, tertiary, neurosyphilis	Penicillin G	All stages; regimen varies	Allergy	Painless chancre → palms/soles → gummas
Borrelia burgdorferi	Lyme disease: erythema migrans, facial palsy, carditis, arthritis	Doxycycline	Early Lyme and many manifestations	Photosensitivity	Ixodes tick + bull's-eye rash
Leptospira	Leptospirosis, Weil disease	Doxycycline; penicillin for severe disease	Spirochetal infection	Doxy: photosensitivity	Rat urine/water + jaundice + renal failure
M. tuberculosis	Pulmonary TB, meningitis, Pott disease, disseminated TB	RIPE	TB	Drug-specific; see below	Acid-fast + apical cavitation
M. leprae	Leprosy	Dapsone + rifampin ± clofazimine	Multibacillary/paucibacillary disease	Dapsone → hemolysis	Cool skin + sensory loss
M. avium complex	Disseminated disease in advanced AIDS	Macrolide-based regimen	Atypical mycobacteria	Drug-specific	AIDS + very low CD4 + disseminated infection

Adverse Effects

Drug/class	🚨 Most important adverse effects	🎯 MCQ clue
Penicillins	Allergy, diarrhea	Anaphylaxis after antibiotic
Cephalosporins	Allergy, diarrhea; some generation-specific effects	β-lactam allergy
Vancomycin	Red-man syndrome, nephrotoxicity	Flushing during rapid IV infusion
Macrolides	QT prolongation, GI upset	Long QT
Tetracyclines	Photosensitivity, teeth/bone discoloration	Avoid in pregnancy/young children
Aminoglycosides	Nephrotoxicity + ototoxicity	Gentamicin/amikacin → kidney + ear
Fluoroquinolones	Tendinitis/tendon rupture, QT prolongation, cartilage toxicity	Achilles tendon
Clindamycin	C. difficile colitis	Severe diarrhea after antibiotic
TMP-SMX	SJS/TEN, hyperkalemia, marrow suppression, G6PD hemolysis	Sulfa + rash/hyperK
Rifampin	Orange body fluids, hepatotoxicity, CYP induction	Orange urine/tears
Isoniazid	Peripheral neuropathy, hepatotoxicity	Give B6/pyridoxine
Pyrazinamide	Hyperuricemia, hepatotoxicity	Gout
Ethambutol	Optic neuritis	Red-green color blindness
Dapsone	Hemolytic anemia, methemoglobinemia	G6PD deficiency
Linezolid	Thrombocytopenia, serotonin syndrome	Long course + SSRI
Daptomycin	Myopathy/rhabdomyolysis	↑ CK; NOT pneumonia
Metronidazole	Metallic taste, disulfiram-like reaction, neuropathy	Alcohol + metronidazole
Chloramphenicol	Aplastic anemia, gray-baby syndrome	Newborn + gray color

ANTIBIOTICS

Mechanisms of antibiotic action

Target	Main drug classes
Cell-wall synthesis	β-lactams, vancomycin
Protein synthesis — 30S	Aminoglycosides, tetracyclines
Protein synthesis — 50S	Macrolides, clindamycin, linezolid, chloramphenicol
DNA replication	Fluoroquinolones
RNA synthesis	Rifampin
Folate synthesis	Sulfonamides, trimethoprim
Cell membrane	Daptomycin; polymyxins

β-lactams

Penicillins

Drug	Typical clinical role
Penicillin V	Selected streptococcal infections
Amoxicillin	Common respiratory and dental indications
Amoxicillin–clavulanate	Broader coverage including selected β-lactamase-producing organisms
Piperacillin–tazobactam	Broad-spectrum hospital infections, including selected Pseudomonas infections

Cephalosporins

Generation	Examples	General pattern
First	Cefazolin, cephalexin	Stronger Gram-positive activity
Second	Cefuroxime, cefoxitin	Expanded selected Gram-negative activity
Third	Ceftriaxone, cefotaxime, ceftazidime	Broader Gram-negative activity; ceftazidime covers Pseudomonas
Fourth	Cefepime	Broad Gram-negative coverage, including Pseudomonas
Fifth	Ceftaroline	Covers MRSA

Coverage varies by individual drug; generation alone does not predict all susceptibility.

Important interactions

Penicillins may cause hypersensitivity reactions.
Methotrexate clearance may be reduced by certain penicillins.
Some cephalosporins can increase bleeding risk in selected settings.
Amoxicillin–clavulanate may cause diarrhea and cholestatic liver injury.

Protein synthesis inhibitors

Class	Examples	Major adverse effects
Aminoglycosides	Gentamicin, amikacin	Nephrotoxicity, ototoxicity
Tetracyclines	Doxycycline, tetracycline	Photosensitivity, GI irritation, tooth/bone effects in developing tissues
Macrolides	Azithromycin, clarithromycin	GI upset, QT prolongation; interactions vary by agent
Clindamycin	Clindamycin	Antibiotic-associated diarrhea and C. difficile colitis
Chloramphenicol	Chloramphenicol	Bone marrow toxicity
Linezolid	Linezolid	Thrombocytopenia, serotonin syndrome risk with serotonergic drugs

Tetracyclines

Bind the 30S ribosomal subunit.
Inhibit aminoacyl-tRNA attachment.
Can discolor developing teeth.
Doxycycline is often preferred over older tetracyclines for certain infections because of pharmacokinetic advantages.

Macrolides

Bind the 50S subunit.
Inhibit translocation.
Clarithromycin and erythromycin inhibit CYP3A4 more substantially than azithromycin.

Dental exam tip: Clindamycin is associated with C. difficile colitis; it should not be used automatically as a substitute for penicillin in every dental infection.

DNA and folate synthesis inhibitors

Drug	Mechanism	Key adverse effect
Fluoroquinolones	Inhibit DNA gyrase/topoisomerase IV	Tendon injury, QT prolongation, CNS effects
Metronidazole	Produces reactive intermediates that damage microbial DNA	GI upset, metallic taste; important interactions with selected drugs
Trimethoprim–sulfamethoxazole	Sequential folate pathway inhibition	Rash, hyperkalemia, marrow suppression
Rifampin	Inhibits bacterial RNA polymerase	Hepatotoxicity, orange body fluids, enzyme induction
Nitrofurantoin	Multiple reactive effects in susceptible bacteria	Pulmonary, hepatic or neuropathic toxicity in selected settings

Tuberculosis — RIPE therapy

Drug	Mechanism	Key adverse effect
Rifampin	Inhibits RNA polymerase	Orange secretions, hepatotoxicity, enzyme induction
Isoniazid	Inhibits mycolic acid synthesis	Hepatotoxicity, peripheral neuropathy
Pyrazinamide	Disrupts mycobacterial metabolism	Hepatotoxicity, hyperuricemia
Ethambutol	Inhibits arabinosyl transferase	Optic neuritis, impaired red-green discrimination

Remember: Give pyridoxine (vitamin B6) with isoniazid when indicated to reduce neuropathy risk, particularly in susceptible patients.