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IV Antibiotics: Mechanism of Action and Coverage

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1. BETA-LACTAMS

Mechanism of Action

Beta-lactams bind the transpeptidase at the PBP site, resulting in inhibition of transpeptidation, thus halting peptidoglycan synthesis. - Katzung 16th Ed

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A. Penicillins (IV)

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B. Cephalosporins (IV)

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C. Carbapenems (IV)

The carbapenems are resistant to most beta-lactamases but not serine carbapenemases nor metallo-beta-lactamases. - Katzung 16th Ed

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D. Monobactams (IV)

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2. GLYCOPEPTIDES (IV)

Mechanism of Action

Vancomycin inhibits cell wall synthesis by binding firmly to the d-Ala-d-Ala terminus of nascent peptidoglycan pentapeptide - Katzung 16th Ed

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3. AMINOGLYCOSIDES (IV)

Mechanism of Action

1. They are taken up across the bacterial cell membrane via an energy-dependent process (requires aerobic respiration - hence inactive against anaerobes and in anaerobic/acidic environments).
2. Inside, they bind to the 30S ribosomal subunit (specifically protein S12 in the case of streptomycin, multiple sites for newer aminoglycosides), causing misreading of mRNA codons - leading to incorporation of wrong amino acids and production of aberrant, non-functional proteins. These abnormal proteins insert into the cell membrane, increasing permeability and allowing more aminoglycoside entry - a self-amplifying process.

The aminoglycoside blocks the normal activity of the "initiation complex" of peptide formation and binds to protein S12 on the 30S subunit - Jawetz Medical Microbiology 28th Ed

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4. FLUOROQUINOLONES (IV)

Mechanism of Action

1. DNA gyrase (topoisomerase II) - primary target in gram-negative bacteria. Introduces negative supercoils ahead of the replication fork.
2. Topoisomerase IV - primary target in gram-positive bacteria. Required for decatenation (separation) of daughter chromosomes after replication.

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5. LINCOSAMIDES & STREPTOGRAMINS (IV)

Mechanism of Action

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6. OXAZOLIDINONES (IV)

Mechanism of Action

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7. LIPOPEPTIDES (IV)

Mechanism of Action

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8. CARBOXYPENICILLINS & TETRACYCLINES - IV

Tigecycline (IV - Glycylcycline)

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9. POLYMYXINS (IV)

Mechanism of Action

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Summary Table

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• MRSA: Vancomycin (1st line), Daptomycin, Linezolid, Ceftaroline
• Pseudomonas: Pip-tazobactam, Ceftazidime, Cefepime, Imipenem/Meropenem, Ciprofloxacin, Aztreonam, Tobramycin - never Ertapenem
• VRE: Linezolid, Daptomycin, Quinupristin-dalfopristin (faecium only)
• CRE (carbapenem-resistant Enterobacterales): Ceftazidime-avibactam, Meropenem-vaborbactam, Colistin, Tigecycline
• Anaerobes: Metronidazole, Pip-tazobactam, Carbapenems, Moxifloxacin, Clindamycin

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How to Remember Gram-Positive vs Gram-Negative Bacteria

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Step 1 - The Core Concept (Never Forget This)

Think: "Gram-Positive = Pretty Purple" and "Gram-Negative = Not Purple (pink)"

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Step 2 - The Master Visual Flowcharts

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Step 3 - The BIG MNEMONIC Framework

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GRAM-POSITIVE COCCI

"Silly Students Sometimes Eat Peanut Butter Sandwiches" S. aureus, S. epidermidis, S. saprophyticus, Enterococcus, Pneumococcus, Beta-strep (Grp A/B), S. viridans

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GRAM-POSITIVE RODS (Bacilli)

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GRAM-NEGATIVE COCCI

Both are diplococci (kidney-bean shaped pairs), oxidase-positive, intracellular.

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GRAM-NEGATIVE RODS (Bacilli) - The Big Group

Enterobacteriaceae ("The Enteric Family")

The "SPACE" Organisms (AmpC beta-lactamase inducers)

• Serratia
• Pseudomonas
• Acinetobacter / Aeromonas
• Citrobacter / C. freundii
• Enterobacter

Non-Enteric Gram-Negative Rods

Spiral/Curved Gram-Negatives

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Step 4 - The "Special" Categories (Neither standard G+ nor G-)

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Step 5 - Quick 60-Second Recall Table

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Step 6 - One Final Super-Mnemonic

• Staph, Strep, Enterococcus (cocci)
• Listeria, Bacillus, Corynebacterium, Clostridium, Actinomyces (rods)

• Enterobacterales: E. coli, Klebsiella, Salmonella, Shigella, Proteus, Enterobacter, Serratia
• Pseudomonas, H. influenzae, Legionella, Francisella, Brucella, Vibrio, Campylobacter

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Anaerobes - Complete Guide

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What Are Anaerobes?

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The Master Classification

ANAEROBES
├── GRAM-POSITIVE
│   ├── Spore-forming RODS → Clostridium spp.
│   ├── Non-spore-forming RODS → Actinomyces, Propionibacterium, Lactobacillus, Bifidobacterium
│   └── COCCI → Peptostreptococcus (Finegoldia magna)
│
└── GRAM-NEGATIVE
    ├── RODS → Bacteroides fragilis group, Prevotella, Porphyromonas, Fusobacterium
    └── COCCI → Veillonella

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GRAM-POSITIVE ANAEROBES

1. Clostridium - The Spore-Formers (Most Important!)

"Big Patients Die Terribly" = Botulinum, Perfringens, Difficile, Tetani

• All gram-positive, spore-forming rods
• Found in soil and GI tract
• Spores survive boiling, autoclaving needed
• C. tetani and C. botulinum are obligate anaerobes; C. perfringens is more aerotolerant

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2. Actinomyces

Mnemonic: "Actino-MY-ces = MINE! Lives in MY jaw, MY lung, MY pelvis"

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3. Peptostreptococcus (Finegoldia magna)

• Gram-positive anaerobic cocci in chains
• Part of normal flora of mouth, GI, vagina
• Causes: mixed wound infections, pelvic abscess, brain abscess, aspiration pneumonia
• Treat with penicillin, clindamycin, metronidazole

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GRAM-NEGATIVE ANAEROBES

1. Bacteroides fragilis - THE Most Clinically Important Anaerobe

Mnemonic: "B. fragilis = FRAG-ile penicillin won't work, use METRO"

From Harrison's 22E: "B. fragilis is the anaerobe most commonly isolated from blood cultures... isolation from blood indicates underlying infection associated with a 60% mortality rate if untreated."

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2. Prevotella & Porphyromonas

Mnemonic: "Pre-VOTT-ella = Previous mouth/throat bug (oro-dental)"

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3. Fusobacterium nucleatum / necrophorum

Mnemonic: "Fuso = Fuse the throat vein shut (Lemierre's)"

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4. Veillonella

• Gram-negative anaerobic cocci (tiny)
• Only gram-negative anaerobic cocci of clinical significance
• Normal flora of the mouth and GI tract
• Rarely pathogenic but found in mixed infections

Mnemonic: "Veil-onella = hidden behind the veil (rarely causes disease alone)"

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WHERE Do Anaerobes Cause Infection? (Clinical Distribution)

From Jawetz 28th Ed: "Clindamycin is preferred for infections above the diaphragm. Relatively few anaerobes are resistant to clindamycin (B. fragilis group excepted) and few, if any, are resistant to metronidazole."

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Clinical Clues That Scream "ANAEROBE!"

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Treatment Summary

NEVER use: Aminoglycosides (inactive - require O₂ uptake), Aztreonam (gram-negative aerobes only), trimethoprim-sulfamethoxazole

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Quick Memory Table - All Key Anaerobes

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The Single Best Mnemonic to Remember All Anaerobes

• Clostridium (4 types: Bot, Perf, Diff, Tet)
• Peptostreptococcus
• E - (Actinomyces → has "es" sound)
• Fusobacterium
• Prevotella / Porphyromonas
• Bacteroides fragilis
• Bifido / other non-spore G+ rods
• Putrid smell = clue to all
• Fragilis = below diaphragm
• Veillonella = GN cocci