Companion to your master table

The antibiotics
field guide.

Every drug your textbook recommends — grouped by class, mapped to bugs, with the hierarchy it gives them. Built so the name ceftriaxone on a page fires cephalosporin → cell-wall → 3rd-gen → DOC for gonorrhoea & meningitis in one beat.

Scope. Drawn only from the treatment sections of Basics in Medical Microbiology and Immunology, Part III (Zagazig, 2013), systematic bacteriology through actinomycetoma. Mycology section excluded as requested. The minimal pharmacology added (one line of mechanism per class + a memory hook) is the only content outside the textbook — exactly enough to make the names stick.

§ 01The five-family map

Every antibiotic in the book lives in one of five mechanism families. Once a drug's family is in your head, its spectrum and toxicity follow logically. Internalise these five and the rest is just sub-categories.

Family 1

Cell wall

Stop peptidoglycan being built. Bacterium bursts from osmotic pressure. β-lactams + glycopeptides. Bactericidal. Useless against wall-less bugs (Mycoplasma).

Family 2

Protein synthesis

Hit the ribosome — 30S (aminoglycosides, tetracyclines) or 50S (macrolides, clindamycin, chloramphenicol, linezolid). Mostly bacteriostatic — except aminoglycosides which are -cidal.

Family 3

Nucleic acids

Sabotage DNA or RNA. Fluoroquinolones jam DNA gyrase. Rifampicin blocks RNA polymerase. Metronidazole creates DNA-damaging radicals (only in anaerobes).

Family 4

Folate synthesis

Starve bacteria of folate (they make their own; we eat ours). Sulfonamides + trimethoprim hit two sequential steps — synergy. Dapsone is a sulfa cousin.

Family 5

Cell membrane / other

Daptomycin punctures the Gram+ membrane. The "last-resort Gram+" drugs (linezolid, synercid, tigecycline) also live here conceptually — drugs you reach for when everything else has failed.

§ 02Reading the drug names

Drug-class suffixes are the cheapest possible memory trick. Five of them cover most of the book:

–cillin	penicillin family (peni-, methi-, oxa-, ampi-, amoxi-, ticar-)
cef–/ceph–	cephalosporins (cefotaxime, ceftriaxone, cefoxitin, cephalothin)
–cycline	tetracyclines (tetra-, doxy-, tige-)
–thromycin	macrolides (ery-, azi-, clari-)
–floxacin	fluoroquinolones (cipro-, ofl-, lev-)

Watch out: –mycin alone is unreliable — gentamicin and streptomycin are aminoglycosides, but vancomycin is a glycopeptide and erythromycin is a macrolide. Use the rest of the stem (gent-, vanco-, eryth-).

§ 03β-Lactams — penicillins, cephalosporins, carbapenems

Family 1 · Cell wall

Penicillins –cillin

Bind penicillin-binding proteins (PBPs) → block peptidoglycan cross-linking → bacterium lyses. Bactericidal. Broken by β-lactamase.

Memory hook Think of the cell wall as a brick wall the bacterium needs to stay together. PBPs are the bricklayers; β-lactams jam their hands. Most resistance is a tiny enzyme (β-lactamase) that snaps the β-lactam ring before it can grab the bricklayer — so the chemistry war is: bigger rings, blocked rings, or smuggled inhibitors (clavulanate / sulbactam) to defend the ring.

Drug	Textbook bugs	Role
Penicillin Gnatural penicillin · IV/IM	Str. pyogenes, Str. agalactiae, viridans, Str. pneumoniae (historical), diphtheria (clears bacilli), tetanus (clears bacilli), gas gangrene (with metronidazole), meningococci, T. pallidum, Borrelia (Lyme & relapsing), Leptospira, actinomycosis (6–12 mo), Bartonella bacilliformis	DOC for Group A strep, syphilis, actinomycosis, Lyme, leptospirosis. COMBO for gas gangrene (+ metronidazole). Adjunct in tetanus/diphtheria (kills bug, antitoxin handles disease).
Benzathine penicillindepot · monthly IM	Group A strep — rheumatic fever prophylaxis	PROPHYLAXIS Monthly injection for years post-RF to prevent recurrence.
Methicillin / Oxacillinβ-lactamase–resistant penicillins	S. aureus (β-lactamase producers — ~90% of clinical isolates)	1st choice for non-MRSA S. aureus. MRSA (~20%) defeats them via altered PBPs → must escalate to vancomycin.
Ampicillinaminopenicillin · oral/IV	Listeria monocytogenes (+ gentamicin), Salmonella (typhoid + carriers), Shigella, severe Yersinia pseudotuberculosis; B. fragilis only when paired with sulbactam	DOC for listeriosis (+ gent in fulminant cases). Used for typhoid (+ MDR concerns), bacillary dysentery. K. pneumoniae is intrinsically resistant.
Amoxicillinaminopenicillin · oral	H. pylori (component of triple therapy)	TRIPLE With PPI + clarithromycin or metronidazole — 1 week regimen.
Ticarcillinextended-spectrum penicillin	B. fragilis (paired with clavulanate)	COMBO Ticarcillin + clavulanate covers β-lactamase-producing anaerobes.
Clavulanate / Sulbactamβ-lactamase inhibitors (not antibiotics alone)	Add-ons that protect any β-lactam from β-lactamase — used with ticarcillin or ampicillin for B. fragilis	ADJUNCT Only useful paired with a real β-lactam. Restores efficacy against β-lactamase producers.

Family 1 · Cell wall

Cephalosporins cef– / ceph–

Same mechanism as penicillins (block PBPs) — but more stable against many β-lactamases. Bactericidal. Five generations; the textbook uses mostly 1st, 2nd & 3rd.

Memory hook Climbing the generations = climbing into the bloodstream and the CSF. 1st gen (cephalothin) — surface Gram-positives only. 2nd gen (cefoxitin) — adds some anaerobic Gram-negatives; that's why it's chosen as preop cover before colon surgery. 3rd gen (cefotaxime, ceftriaxone) — crosses the BBB, kills the meningitis bugs (meningococci, H. influenzae) and gonorrhoea.

Drug	Textbook bugs	Role
Cephalothin1st gen	Yersinia — listed only as resistant (PenG, ampicillin, cephalothin all fail)	Resistance marker Mentioned to flag that 1st-gen cephs don't cover Yersinia.
Cefoxitin2nd gen (cephamycin)	Bacteroides fragilis — surgical prophylaxis	PRE-OP Recommended preoperatively to prevent Bacteroides contamination of surgical wounds.
Cefotaxime / Ceftriaxone3rd gen · IV · cross BBB	Meningococcal meningitis, H. influenzae type b meningitis, gonorrhoea, H. ducreyi chancroid, severe Yersinia infections	DOC for gonorrhoea (ceftriaxone) and a meningitis first-line. COMBO with azithromycin in chancroid.
3rd-gen cephalosporinsclass mention	Salmonella (typhoid), Shigella (bacillary dysentery)	Used alongside fluoroquinolones for MDR enterics.

Family 1 · Cell wall

Carbapenems –penem

β-lactam with the broadest spectrum of any class. Resistant to almost all β-lactamases. Bactericidal.

Memory hook The β-lactam nuclear option. In the textbook it appears for one bug only: B. fragilis — when β-lactamase has knocked out the simpler penicillins and the patient still has a deep anaerobic infection.

Drug	Textbook bugs	Role
Imipenem	Bacteroides fragilis	Effective Listed as one of the β-lactams that works against β-lactamase-producing B. fragilis.

§ 04Glycopeptides

Family 1 · Cell wall (different target)

Glycopeptides vanco–

Bind D-Ala–D-Ala on peptidoglycan precursors → block cross-linking from a different angle than β-lactams. Bactericidal. The molecule is too bulky to cross the outer membrane of Gram-negatives — that's why vancomycin is a Gram-positive-only drug.

Memory hook "When all –cillins fail, call the VAN." Vancomycin is the cavalry for resistant Gram-positives: MRSA, severe S. epidermidis implant infections, β-lactam-resistant pneumococci, and C. difficile colitis (oral, where it stays in the gut). VRE = vancomycin-resistant enterococci = the one Gram-positive battle vancomycin has lost.

Drug	Textbook bugs	Role
VancomycinIV systemic · ORAL only for C. diff	MRSA / ORSA S. aureus; S. epidermidis (DOC, plus rifampicin or gentamicin); penicillin-resistant S. pneumoniae ("the best"); oral therapy of C. difficile pseudomembranous colitis	DOC for MRSA and S. epidermidis. "Best" per textbook for resistant pneumococci. Alt. to metronidazole for C. difficile.

§ 05Aminoglycosides

Family 2 · Protein synthesis (30S)

Aminoglycosides gent– · strep– · kana–

Bind 30S ribosomal subunit → cause misreading of mRNA → faulty proteins puncture the membrane. Bactericidal (rare for a protein-synthesis drug). Don't penetrate intact Gram-positive walls well — that's why they're almost always paired with a cell-wall drug.

Memory hook The eternal aminoglycoside partnership: β-lactam punches the hole, aminoglycoside walks in. Every aminoglycoside use in the book is a synergistic combo — ampicillin + gentamicin for Listeria, β-lactam + aminoglycoside for Pseudomonas, vancomycin + gentamicin for S. epidermidis, tetracycline + streptomycin for Brucella. Never solo.

Drug	Textbook bugs	Role
Gentamicin	Listeria monocytogenes (+ ampicillin), P. aeruginosa serious infections (+ β-lactam), S. epidermidis (+ vancomycin), severe Yersinia infections, Y. pestis plague	COMBO Always paired. Never first-line monotherapy in the textbook.
Streptomycin	Brucella (+ tetracycline), Y. pestis plague, TB (1st line), Bartonella bacilliformis Oroya fever	COMBO for brucellosis. 1st-line in anti-TB regimen.
Kanamycin	M. tuberculosis — drug-resistant strains	2nd-line TB Reserved for treatment failure or 1st-line resistance.

§ 06Tetracyclines & glycylcyclines

Family 2 · Protein synthesis (30S)

Tetracyclines –cycline

Bind 30S → block aminoacyl-tRNA from docking → no peptide elongation. Bacteriostatic. Penetrate cells well — that's the secret to their oddball spectrum.

Memory hook "Tetracyclines hunt the intracellular and the atypical" — they go where β-lactams can't. Mental list of tetracycline bugs: Rickettsia · Chlamydia · Mycoplasma · Brucella · Borrelia · Leptospira · Vibrio cholerae · Helicobacter pylori · Coxiella. Notice the pattern — almost every one is either a wall-less, atypical, or intracellular organism. Tetracyclines are the textbook's weird-bug drug.

Drug	Textbook bugs	Role
Tetracyclineparent compound	V. cholerae (shortens carriage), H. pylori (triple therapy option), Brucella (+ strep or rif), T. pallidum (penicillin allergy), Borrelia (Lyme, relapsing), Leptospira, Mycoplasma, Rickettsia group (typhus), Bartonella henselae; mentioned as resistance in B. fragilis	DOC for Rickettsia, Mycoplasma. Pen-allergy alt for syphilis. COMBO for brucellosis & H. pylori.
Doxycyclinelong-acting tetracycline	B. anthracis anthrax, B. cereus, Y. pestis plague (+ aminoglycoside for severe), Y. enterocolitica, R. typhi, R. rickettsii, Orientia tsutsugamushi, Coxiella burnetii Q fever, Chlamydia STD	DOC for Q fever, scrub typhus, Rocky Mountain spotted fever, anthrax (with ciprofloxacin as the other option). DOC for chlamydial STDs (alongside azithromycin).

Family 5 · Last-resort Gram-positive

Glycylcyclines tige–

Structurally modified tetracycline — same 30S target, but evades the efflux pumps and ribosomal protection that defeat older tetracyclines. Bacteriostatic, broad-spectrum.

Memory hook Tetracycline's "tiger" cousin — bigger jaws, eats the bugs old tetracyclines couldn't (including VRE). The textbook mentions it only as one of the "newer antibiotics" against VRE.

Drug	Textbook bugs	Role
Tigecycline	Vancomycin-resistant enterococci (VRE)	Newer Listed alongside synercid, linezolid, daptomycin for VRE.

§ 07Macrolides

Family 2 · Protein synthesis (50S)

Macrolides –thromycin

Bind 50S subunit → block translocation of growing peptide chain. Bacteriostatic. Concentrate inside cells — like tetracyclines, they reach intracellular bugs.

Memory hook Two stories cover almost every macrolide use in the book: (1) atypical pneumonia / wall-less bugs — Mycoplasma, Legionella, Chlamydia, also Bordetella & Campylobacter; (2) penicillin-allergic patients — Group A strep, syphilis, diphtheria, actinomycosis. The three siblings divide labour: erythromycin is the old workhorse, azithromycin has a long half-life (single-dose treatments), clarithromycin earns its place in H. pylori triple therapy.

Drug	Textbook bugs	Role
Erythromycin	Str. pyogenes (pen allergy), diphtheria (clears bacilli & contacts), Bordetella pertussis, B. cereus, Campylobacter jejuni, atypical pneumonia (Mycoplasma, Legionella, Chlamydia), syphilis (pen allergy), neonatal gonococcal conjunctivitis (ointment prevention), Bartonella, actinomycosis (pen allergy)	DOC for whooping cough. Pen-allergy alt across many strep/spirochete/Gram+ infections. Prophylaxis for pertussis contacts and neonatal gonococcal eye infection.
Azithromycinlong t½ · often single-dose	N. gonorrhoeae co-treatment (covers concurrent Chlamydia), H. ducreyi chancroid (+ ceftriaxone), Chlamydia STD, Legionella, Mycoplasma	DOC for chlamydial STDs (single dose, alongside doxycycline). COMBO with ceftriaxone for chancroid.
Clarithromycin	Helicobacter pylori	TRIPLE PPI + amoxicillin + clarithromycin, 1 week regimen.

§ 08Lincosamides

Family 2 · Protein synthesis (50S)

Lincosamides clinda–

Bind 50S at the same site as macrolides → block peptide bond formation. Bacteriostatic. Penetrates abscesses and bone well — that's its niche.

Memory hook "Clindamycin = Anaerobes." Treats the same anaerobic niche metronidazole owns, plus serves as a penicillin-allergy substitute for actinomycosis. Ironic risk: by wiping out anaerobic flora, it's one of the classic triggers of C. difficile colitis — the very thing it can also be used to treat in different settings.

Drug	Textbook bugs	Role
Clindamycin	Bacteroides fragilis; actinomycosis (penicillin-allergic patients)	Used for B. fragilis ("most strains susceptible"). Pen-allergy alt for actinomycosis.

§ 09Chloramphenicol

Family 2 · Protein synthesis (50S)

Chloramphenicol chlor–

Binds 50S → blocks peptidyl transferase. Bacteriostatic, broad-spectrum. Limited by the well-known toxicity (bone-marrow suppression, "grey baby syndrome") — so reserved.

Memory hook "Broad but banned by the bone marrow." The textbook only reaches for chloramphenicol when nothing else works: meningococcal meningitis where resistance is suspected, severe Rickettsiae, complex Yersinia, Salmonella, Bartonella. Think of it as the "glass-break in case of emergency" antibiotic.

Drug	Textbook bugs	Role
Chloramphenicol	Meningococci (when β-lactam resistance suspected), Salmonella (typhoid), severe Yersinia, R. prowazekii epidemic typhus, R. typhi endemic typhus, Bartonella bacilliformis	Reserve Used when 1st-line options fail or are contraindicated.

§ 10Fluoroquinolones

Family 3 · Nucleic acids

Fluoroquinolones –floxacin

Inhibit bacterial DNA gyrase (topoisomerase II) and topoisomerase IV → DNA can't unwind for replication. Bactericidal. Excellent oral bioavailability, broad-spectrum, penetrates secretions (saliva, prostate).

Memory hook Two stories: (1) Gram-negative enterics in the gut — Salmonella, Shigella, Campylobacter, Yersinia; (2) bioterror & bug-on-the-edge cases — anthrax, plague, B. cereus, 2nd-line TB. Plus one special trick: ciprofloxacin is secreted into saliva — that's why it's a meningococcal chemoprophylaxis drug (kills the carriage in the nasopharynx of close contacts), where penicillin can't go.

Drug	Textbook bugs	Role
Ciprofloxacin	B. anthracis anthrax, B. cereus, Salmonella (chronic carriers), Shigella bacillary dysentery, Y. pestis plague, Y. enterocolitica, Campylobacter jejuni, meningococcal chemoprophylaxis, MDR/XDR-TB (2nd line)	DOC for anthrax. Chemoprophylaxis for meningococcal contacts (saliva secretion). 2nd-line TB.
Fluoroquinolonesclass mention — implies cipro, levo, etc.	Penicillin-resistant S. pneumoniae, Salmonella, severe Yersinia, Legionella Legionnaires' disease	First-line alternative for Legionella alongside macrolides. Effective against penicillin-resistant pneumococci.

§ 11Rifamycins

Family 3 · Nucleic acids

Rifamycins rifa–

Block bacterial DNA-dependent RNA polymerase → no transcription, no proteins. Bactericidal. Penetrates everywhere — abscesses, CSF, intracellular bugs, even saliva.

Memory hook Three things to remember about rifampicin: (1) Red urine, tears, sweat — harmless, just the drug colour; (2) Resistance emerges fast if given alone → that's why every textbook use of it is a combination (with INH for TB, with dapsone for leprosy, with tetracycline for brucellosis, with vancomycin for S. epidermidis); (3) Real penetration — secreted into saliva, like ciprofloxacin, so it's the other meningococcal prophylaxis drug (and used to clear H. influenzae nasal carriage).

Drug	Textbook bugs	Role
Rifampicinaka rifampin	M. tuberculosis (1st-line, "the second major" antitubercular), M. leprae (+ dapsone), Brucella (+ tetracycline, alt. to streptomycin), S. epidermidis (+ vancomycin), meningococcal chemoprophylaxis (close contacts), H. influenzae nasal carriage	1st-line TB. COMBO in every use case (never solo — resistance). Chemoprophylaxis for meningococcal contacts.

§ 12Nitroimidazoles

Family 3 · Nucleic acids

Nitroimidazoles metro–

A prodrug: only the reducing environment inside anaerobes activates it. Once activated, it generates radicals that fragment bacterial DNA. Bactericidal — but only in anaerobes / microaerophilic bugs.

Memory hook "Metro runs only on the anaerobic line." The drug literally cannot work in oxygen-rich tissue — it needs the bacterium's own reducing chemistry to switch on. So its bug list is a list of anaerobes and microaerophiles: Bacteroides, C. difficile, C. perfringens (gas gangrene, with penicillin), Gardnerella (bacterial vaginosis), and the microaerophilic H. pylori.

Drug	Textbook bugs	Role
Metronidazole	C. difficile pseudomembranous colitis (oral), C. perfringens gas gangrene (+ penicillin), B. fragilis, H. pylori (triple therapy), Gardnerella vaginalis bacterial vaginosis	DOC for C. difficile and bacterial vaginosis. DOC for B. fragilis ("most strains susceptible"). COMBO for gas gangrene & H. pylori.

§ 13Folate-synthesis inhibitors

Family 4 · Folate synthesis

Sulfonamides + Trimethoprim SXT / co-trimoxazole

Bacteria must make their own folate (we eat ours). Sulfa blocks PABA → DHF; Trimethoprim blocks DHF → THF. Two sequential blocks of the same pathway = synergy, bactericidal in combination.

Memory hook Two cuts on the same assembly line. Alone, each is bacteriostatic; together (as TMP-SMX / SXT / co-trimoxazole), they're bactericidal. Textbook signature use: DOC for Nocardia — and that's the headline fact worth pinning. Otherwise an alternative for Listeria, Salmonella carriers, and Yersinia.

Drug	Textbook bugs	Role
Trimethoprim-sulfamethoxazoleSXT · TMP-SMX · co-trimoxazole	Nocardia (DOC), Listeria monocytogenes (with ampicillin), Salmonella typhoid & chronic carriers, severe Yersinia infections	DOC for nocardiosis. Alternative for listeriosis, typhoid, complex Yersinia.

Family 4 · Folate synthesis

Sulphones dap–

Structural cousin of sulfonamides — same folate-blocking mechanism, with a special place in mycobacterial therapy.

Memory hook "Dapsone = Leprosy backbone." Always paired with rifampicin (dodging resistance), continued until skin lesions are organism-free — typically 2 years or more.

Drug	Textbook bugs	Role
Dapsonesulphone	M. leprae — leprosy	Main agent for leprosy, combined with rifampicin to prevent resistance.

§ 14Last-resort Gram-positive arsenal

The textbook lumps these together as "newer antibiotics" for MRSA and VRE — bugs that have defeated vancomycin or the wider Gram-positive arsenal. Different classes, same purpose.

Family 2 · Protein synthesis (50S)

Oxazolidinones linez–

Bind 50S at a unique site → block initiation of protein synthesis (no other class does this). Bacteriostatic.

Memory hook "LINE-Z-olid = the LAST LINE for resistant Gram-positives." Reserved for MRSA & VRE. Oral and IV equally bioavailable.

Drug	Textbook bugs	Role
Linezolid	MRSA S. aureus, vancomycin-resistant enterococci (VRE)	Newer Listed as available for resistant strains.

Family 2 · Protein synthesis (50S)

Streptogramins synercid

Two drugs that synergise on the 50S — together they're bactericidal; each one alone is only bacteriostatic.

Memory hook "SYNercid = SYNergistic killing of VRE." Quinupristin + dalfopristin in a fixed combo — the name itself encodes the chemistry.

Drug	Textbook bugs	Role
Quinupristin/dalfopristinSynercid · fixed combination	MRSA, VRE	Newer For multidrug-resistant Gram+ infections.

Family 5 · Cell membrane

Lipopeptides dapto–

Insert into the Gram-positive cell membrane → form pores → depolarisation → rapid death. Bactericidal. Doesn't work in lung tissue (inactivated by surfactant — a useful trivia point).

Memory hook "DAPtomycin = DAPs holes in the Gram-positive membrane." Same target environment as vancomycin — when vanco fails, daptomycin is one of the next moves.

Drug	Textbook bugs	Role
Daptomycin	MRSA S. aureus, VRE	Newer Listed for resistant Gram-positive infections.

§ 15Anti-tubercular drugs

Anti-mycobacterial · multi-class

First-line TB drugs RIPES

A multidrug regimen is mandatory — single-drug therapy guarantees resistance because M. tuberculosis divides slowly and persists intracellularly. Duration: 6–12 months. Sputum becomes non-infectious within 2–3 weeks of starting.

Memory hook RIPES — every TB doctor's first word: Rifampicin · Isoniazid · Pyrazinamide · Ethambutol · Streptomycin. The textbook calls isoniazid "the major antituberculous" and rifampicin "the second major." Those two together define MDR-TB: lose them both, and you've crossed into multidrug-resistant territory.

Drug	Mechanism (one line)	Role
Isoniazid (INH)	Blocks mycolic acid synthesis (the wax in the mycobacterial wall)	1st · "the major"
Rifampicin	Blocks bacterial RNA polymerase (see § 11)	1st · "the second major"
Pyrazinamide	Activated in the acidic phagolysosome — kills intracellular dormant bacilli	1st
Ethambutol	Blocks arabinogalactan synthesis (cell wall component)	1st
Streptomycin	Aminoglycoside — 30S misreading (see § 05)	1st

Anti-mycobacterial · reserve

Second-line TB drugs resistance / failure

More toxic, less effective. Brought in only when first-line drugs fail or resistance has emerged.

Drug	Class / mechanism	Role
Ethionamide	Structural analogue of INH — blocks mycolic acid synthesis	2nd-line
Kanamycin	Aminoglycoside (30S)	2nd-line
Capreomycin	Polypeptide — inhibits protein synthesis	2nd-line
Cycloserine	Blocks D-alanine incorporation into peptidoglycan (cell wall)	2nd-line
Fluoroquinolonese.g. ciprofloxacin	DNA gyrase inhibitor (see § 10)	2nd-line

§ 16Topical agents

Topical · skin / nasal

Mupirocin mupi–

Inhibits bacterial isoleucyl-tRNA synthetase → no protein synthesis. Used topically because it's inactivated quickly when systemic.

Memory hook The textbook mentions it once, with a clear job: intranasal mupirocin to reduce persistent S. aureus nasal colonisation. Complete eradication is hard, but reduction is achievable.

Drug	Textbook use	Role
Mupirocinintranasal ointment	S. aureus — nasal decolonisation	Topical

§ 17Clinical-syndrome panels

The textbook builds five combination regimens worth memorising as whole patterns — not just lists of drugs. These come up repeatedly in exam questions.

M. tuberculosis — first-line regimen

Rifampicin — "the second major"

Isoniazid — "the major antituberculous"

Pyrazinamide

Ethambutol

Streptomycin

Duration: 6–12 months
Sputum becomes non-infectious: 2–3 weeks after start
MDR-TB: resistance to isoniazid and rifampicin (the two majors)
XDR-TB: MDR + resistance to any fluoroquinolone + ≥3 second-line drugs
Why multi-drug? Single-drug therapy guarantees resistance during the long treatment duration

H. pylori — triple therapy (1–2 weeks → 95% eradication)

Acid suppression — proton pump inhibitor (also inhibits H. pylori urease & enhances ulcer healing)

Amoxicillin (β-lactam)

Metronidazole OR Clarithromycin (the third agent)

Alternative regimen: Bismuth salt + metronidazole + (amoxicillin OR tetracycline) for 14 days
Why three drugs? Eradication rates with single drugs are too low because H. pylori sits in mucus and is hard to reach

MRSA / VRE — the resistant-Gram+ arsenal

MRSA S. aureus: vancomycin (DOC). Newer options: linezolid, daptomycin, quinupristin/dalfopristin.
S. epidermidis: vancomycin + (rifampicin OR gentamicin). Especially important in prosthetic/catheter infections.
VRE (vancomycin-resistant enterococci): quinupristin/dalfopristin · linezolid · daptomycin · tigecycline.
Pen-resistant S. pneumoniae: New cephalosporins, fluoroquinolones — but textbook says "vancomycin is the best."

Anaerobic infections — the metronidazole + clindamycin domain

B. fragilis: Mainstay: drainage + debridement. Antibiotics: clindamycin or metronidazole (most strains susceptible). Reserves: imipenem, ticarcillin + clavulanate, ampicillin + sulbactam. Pre-op cover: cefoxitin. Note: resistant to most cephalosporins and tetracyclines.
Gas gangrene (C. perfringens): Surgery + hyperbaric O₂ + penicillin + metronidazole.
C. difficile: Step 1: stop the offending antibiotic (cephalosporins / FQs / clindamycin / β-lactam-inhibitors / macrolides are the classic triggers). Step 2: oral metronidazole or vancomycin.

Penicillin-allergy substitutes (recurring textbook pattern)

Group A strep (Str. pyogenes): erythromycin
Syphilis (T. pallidum): tetracycline or erythromycin
Actinomycosis: clindamycin or erythromycin
The pattern: macrolides are the default penicillin substitute when an oral, broad Gram-positive option is needed. Tetracyclines step in for spirochetes.

§ 18Bug → Drug cross-index

Reverse lookup, grouped by the same categories as your master table. Roles shown only where the textbook is explicit. DOC = drug of choice · 1st = first-line · ALT = alternative · COMBO = always part of a combination · CHEMO = chemoprophylaxis · 2nd = second-line.

Gram-positive cocci

Organism	Drugs (textbook-stated hierarchy)
S. aureus methicillin-susceptible (~90%)	1st Methicillin / oxacillin · cephalosporins · vancomycin
S. aureus MRSA / ORSA	DOC Vancomycin · Newer: linezolid, daptomycin, quinupristin/dalfopristin · Nasal carriage: mupirocin
S. epidermidis	DOC Vancomycin + + rifampicin OR gentamicin
Str. pyogenes Group A β-haemolytic	DOC Penicillin · Pen-allergy: erythromycin · RF prophylaxis: benzathine penicillin (monthly)
Str. pneumoniae	Historical: penicillin (now resistance prevalent) · Effective: new cephalosporins, fluoroquinolones · "Best": vancomycin
Enterococci VRE	Resistant to penicillins & aminoglycosides. Newer: quinupristin/dalfopristin · linezolid · daptomycin · tigecycline

Gram-negative cocci — Neisseriae

N. gonorrhoeae	DOC Ceftriaxone + tetracycline OR azithromycin (cover concurrent Chlamydia, 50% co-infection) · Newborn eye prophylaxis: erythromycin ointment
N. meningitidis	1st IV penicillin G or ceftriaxone (immediately) · Resistance: chloramphenicol · Chemoprophylaxis of contacts: rifampicin OR ciprofloxacin (saliva-penetrating)

Gram-positive bacilli — aerobic

Corynebacterium diphtheriae	Critical: diphtheria antitoxin (neutralises circulating toxin) · + penicillin OR erythromycin (clears bacilli) · Contacts: erythromycin / long-acting penicillin
Listeria monocytogenes	DOC Ampicillin + gentamicin · Alt: ampicillin + SXT · Gastroenteritis: usually no treatment
Bacillus anthracis	DOC Ciprofloxacin or doxycycline
Bacillus cereus	Resistant to penicillin (β-lactamase). Alternatives: doxycycline, erythromycin, ciprofloxacin

Gram-positive bacilli — anaerobic (Clostridia)

C. tetani	Primary: human tetanus immune globulin + booster toxoid · + penicillin (clears bacilli) · wound care · muscle relaxants
C. perfringens gas gangrene	Mainstay: surgical debridement + hyperbaric O₂ · + penicillin + metronidazole
C. difficile	Step 1: stop offending antibiotic · Step 2: oral metronidazole OR vancomycin

Gram-negative bacilli — Enterobacteriaceae

E. coli extra-intestinal	Empirical: guided by AST due to resistance. Diarrhoea: rehydration ± selected antibiotics
Klebsiella pneumoniae	Intrinsically resistant to ampicillin. ESBL → resists most cephalosporins → routine in vitro AST required
Proteus spp.	Per AST — resistant to many drugs
Salmonella typhoid / enteric fever	1st: chloramphenicol · ampicillin · SXT · fluoroquinolones · 3rd-gen cephalosporins (per AST — MDR is the rule). Chronic carriers: ampicillin · SXT · ciprofloxacin (cholecystectomy if gall bladder disease)
Shigella	Mainstay: fluid + electrolyte replacement · + ampicillin · ciprofloxacin · 3rd-gen cephalosporins (shortens illness)

Gram-negative bacilli — non-fermenters

Pseudomonas aeruginosa	Resistant to many drugs → AST required. Serious infections: β-lactam + aminoglycoside
Acinetobacter spp.	Often highly resistant. AST-guided therapy only

Curved Gram-negative bacilli

Vibrio cholerae	Mainstay: IV fluid + electrolyte correction · + tetracycline (shortens shedding)
Campylobacter jejuni	Rehydration ± erythromycin or ciprofloxacin (severe cases only)
Helicobacter pylori	TRIPLE THERAPY: PPI + amoxicillin + (clarithromycin OR metronidazole) for 1 week. Alt: bismuth + metronidazole + (amoxicillin OR tetracycline) × 14 days

Fastidious / zoonotic Gram-negatives

Haemophilus influenzae type b meningitis	DOC Cefotaxime OR ceftriaxone · + steroids 15–20 min pre-antibiotic (limits inflammatory neurologic damage) · Nasal carriage / contacts: rifampicin
Haemophilus ducreyi chancroid	Macrolide (azithromycin) + 3rd-gen cephalosporin (ceftriaxone)
Bordetella pertussis	DOC Erythromycin · Exposed unimmunized: erythromycin
Brucella spp.	Prolonged combo: tetracycline + (streptomycin OR rifampicin)
Yersinia pestis plague	Early therapy critical: tetracyclines (doxycycline) · fluoroquinolones (ciprofloxacin) · aminoglycosides (streptomycin / gentamicin)
Yersinia enterocolitica	Mild: self-limiting. Severe: doxycycline + aminoglycoside · alts: SXT · ceftriaxone · FQs · chloramphenicol. Resistant to penicillin G, ampicillin, cephalothin.
Bacteroides fragilis	Drainage + debridement primary. Most strains: clindamycin OR metronidazole. β-lactam reserves: imipenem · ticarcillin/clavulanate · ampicillin/sulbactam. Pre-op: cefoxitin. Resistant: most cephs, tetracycline
Legionella pneumophila	DOC Macrolides (erythromycin, azithromycin) OR fluoroquinolones

Mycobacteria

M. tuberculosis	1st-line: Isoniazid + Rifampicin + Pyrazinamide + Ethambutol + Streptomycin (RIPES) · 6–12 months · 2nd-line: ethionamide · kanamycin · capreomycin · cycloserine · fluoroquinolones
M. leprae	Main agent: dapsone + rifampicin (prevent resistance) · until lesions organism-free (≥ 2 years)

Spirochetes

Treponema pallidum syphilis	DOC Penicillin · Pen-allergy: tetracycline OR erythromycin
Borrelia burgdorferi Lyme disease	Tetracyclines · penicillin
Borrelia recurrentis & B. duttoni relapsing fever	Penicillin · tetracyclines
Leptospira interrogans	Penicillin · tetracyclines

Wall-less & obligate intracellular

Mycoplasma spp.	Tetracyclines · macrolides (erythromycin, azithromycin). β-lactams useless (no cell wall)
Rickettsia prowazekii epidemic typhus	Tetracycline · chloramphenicol
Rickettsia typhi endemic typhus	Doxycycline · chloramphenicol
Rickettsia rickettsii RMSF	Doxycycline
Orientia tsutsugamushi scrub typhus	Doxycycline
Coxiella burnetii Q fever	DOC Doxycycline
Chlamydia trachomatis STD serotypes D–K	DOC Doxycycline OR azithromycin · treat partners
Chlamydophila psittaci / pneumoniae	Tetracycline · erythromycin

Minor pathogens & fungus-like bacteria

Gardnerella vaginalis bacterial vaginosis	DOC Metronidazole
Bartonella bacilliformis Oroya fever	Penicillin · streptomycin · chloramphenicol
Bartonella henselae cat-scratch disease	Supportive primarily · tetracycline or erythromycin may help
Actinomyces actinomycosis	DOC Surgical drainage + penicillin G × 6–12 months · Pen-allergy: clindamycin OR erythromycin
Nocardia nocardiosis	DOC Trimethoprim-sulfamethoxazole
Actinomycotic mycetoma	Surgical drainage + debridement + antimicrobials (responds well, unlike eumycotic)

§ 19Abbreviations

DOC Drug of choice

1st / 2nd First / second-line

ALT Alternative (e.g. pen-allergy)

COMBO Combination required

CHEMO Chemoprophylaxis

EMP Empirical (AST-guided)

TOP Topical

AST Antibiotic susceptibility testing

MIC Minimum inhibitory concentration

PBP Penicillin-binding protein

ESBL Extended-spectrum β-lactamase

MRSA Methicillin-resistant S. aureus

ORSA Oxacillin-resistant S. aureus

VRE Vancomycin-resistant enterococci

MDR-TB Resistance to INH + RIF

XDR-TB MDR + FQ + ≥3 second-line

INH Isoniazid

RIF Rifampicin

SXT / TMP-SMX Trimethoprim-sulfamethoxazole

FQ Fluoroquinolone

PPI Proton pump inhibitor

30S / 50S Ribosomal subunits

BBB Blood-brain barrier

STD Sexually transmitted disease

RF Rheumatic fever

RMSF Rocky Mountain spotted fever