Antibiotics That Alter The Cell Wall

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Penicillin

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- bacteriocidal by inhibiting the ability of bacteria to synthesize the cell wall (peptidoglycan layer)
- They are most effective in killing actively reproducing bacteria.
- Bacteria that become resistant to penicillin antibiotics do so by synthesizing an enzyme called penicillinase, which destroys the drug.
- Penicillin antibiotics are categorized into four classes.

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Classes of Penicillin

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Acid labile
Acid stable
Penicillinase resistant
Extended spectrum

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acid labile

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- Prototypes: Penicillin G, penicillin G procaine, penicillin G benthazine
- Susceptible organisms: Gram+ cocci
- Comments: Destroyed by gastric acid. Given IV or IM. High sodium content; may cause hypernatremia. Procaine formulation reduces renal elimination, has long duration. Benzathine "depot" formulation given IM lasts for weeks. Avoid skin contact to reduce risk of allergic sensitization.

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acid stable

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- Prototypes: Penicillin V, ampicillin, amoxicillin
- Susceptible organisms: Gram+ cocci. Ampicillin is also effective against some gram– rods
- Comments: Can be taken orally. Ampicillin and amoxicillin are safe in pregnancy

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penicillinase resistant

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- Prototypes: Methicillin, cloxacillin, oxacillin, naficillin, floxacillin, dicloxacillin
- Susceptible organisms Penicillinase-producing staphylococci
- Comments: Less effective against non–penicillinase-producing bacteria. Do not penetrate staphylococcal abscesses well. Do penetrate bone well. High sodium content; may cause hypernatremia. Resistant organisms now common (eg, MRSA)

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extended spectrum

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- Prototypes: Carbenicillin
Susceptible organisms: Similar to ampicillin. Also Pseudomonas aeruginosa.
- Comments: High sodium content; may cause hypernatremia. May also cause neutropenia or bleeding. Drug concentrates in urine; decrease dose in renal impairment.

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Cephalosporins

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- have the same mechanism of action as penicillins: they destroy the bacterial cell wall.
- They can share allergic cross-sensitivity with penicillins, but the risk (0.5%) is actually
- classified into three "generations." First-generation cephalosporins have the same efficacy against gram+ cocci as amoxicillin. Importantly, second-generation cephalosporins have less activity against gram+ cocci than do first-generation agents. Many newer third-generation cephalosporins are very broad spectrum and are marketed for a variety of common infections that can be effectively and inexpensively treated with amoxicillin or first-generation cephalosporins.

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first generatio

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- Prototypes: Cefazolin, cefadroxil, cephalexin
Susceptible organisms: Gram+ cocci. Some gram– rods
- Comments: Less nephrotoxic than 2nd, 3rd generation. Ineffective against penicillin-resistant organisms.

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second generation

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- Prototypes: Cefotoxin, cefaclor, cefamandole, cefluoxime, cefonocid
- Susceptible organisms: Less activity against gram+ cocci than first generation. Should not be used to treat streptococcal pharyngitis. Expanded activity against gram– organisms
- Comments: Ineffective against penicillin-resistant organisms. Cefaclor penetrates middle ear well; used for amoxicillin-resistant otitis media due to Haemophilus. Cefamandole, cefluoxime, and cefonocid are effective in susceptible strains of community-acquired Streptococcus pneumoniae.

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third generation

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Prototypes: Cefotaxime, ceftriaxone
- Susceptible organisms: Much less activity against gram+ cocci than first generation. Expanded activity against gram– bacilli. Not effective against anaerobic bacteria.
Comments: Ineffective against penicillin-resistant organisms. Good penetration into CSF through inflamed meninges. Ceftriaxone is the first-line antibiotic for suspected cases of bacterial meningitis. A single IM dose of ceftriaxone (Rocephin) is effective against Neisseria gonorrhoeae and otitis media. Hiccups and gallbladder symptoms are common side effects.

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newer third generation

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- Prototypes: Cefpodoxime (Vantin), cefixime (Suprax), cefdinir (Omnicef)
- Susceptible organisms: Very broad spectrum against gram+ cocci and gram– organisms
Comments: Once-a-day dosing is appealing. Increased risks to patient and community from using broad-spectrum antibiotics.

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Vancomycin

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- a drug that is reserved for IV therapy of (MRSA.
- In the past, it was a first-line oral drug for treating pseudomembranous colitis due to Clostridium difficile. Because it is not orally absorbed, vancomycin given PO concentrates in the colon. The CDC no longer recommends vancomycin for treating "C. diff" because of the emergence of a superinfection called vancomycin-resistant Enterococcus (VRE). Metronidazole (Flagyl) is now the drug of choice for treating pseudomembranous colitis. VRE is also becoming more common in patients treated with vancomycin for MRSA. Other points about vancomycin are the following:
- Adverse drug effects include ototoxicity and nephrotoxicity.
- Never give vancomycin IM, as it is a local irritant.
- The IV infusion rate must be carefully controlled to avoid histamine release and subsequent serious hypotension and "red neck syndrome," which can lead to cardiac arrest. The protocols at some hospitals the administration of an antihistamine to prevent histamine release so that vancomycin can be infused more rapidly.