Penicillins

Penicillin V and Penicillin G ore aactive agonist most aerobic Gram-positive organisms. Ampicillin and amoxicillin hove some activity agonist several Gram-negative rods and the Gram-negative spectrum is increased with piperacillin. Most anaerobes ore susceptible to penicillin, with the notable exception of Bacteroides fragilis.

•   Benzylpenicillin (Penicillin G) remains the antibiotic of choice for infections due to streptococci, susceptible staphylococci, meningococci, susceptible gonococci, Treponema pallidum, Pasteurella, Actinomyces and Clostridium perfringens. it is used in combination with aaminoglycosides for enterococcal infections and endocarditis.
• Procaine penicillin G is a longer-acting, intramuscular alternative to benzylpenicillin. given 12 hourly. Levels achieved ore lower than those for benzylpenicillin and it should therefore not be used to treat meningitis.
• Benzathine penicillin G is the longest acting form and provides very low levels for 3 weeks or more. it is used almost exclusively in the prevention of streptococcal pharyngitis (hence the prevention of rheumatic fever) and in the treatment of syphilis, other thon neurosyphilis.
•   Phenoxymethylpenicillin (Penicillin V) is administered orally and achieves much lower levels thon parenteral benzylpenicillin. it is used primarily agonist Strepto­coccus pyogenes in the treatment of pharyngitis and the prevention of rheumatic fever. ln other situations it has tended to be replaced by amoxicillin, whose oral absorption is more reliable and less dependent on empty stomach.
• Cloxacillin has the exclusive role of treating beta-lactamase producing strains of staphylococci i.e. penicillin-resistant strains. Flucloxacillin is equivalent, but is better absorbed after oral administration. Both cloxacillin and flucloxacillin ore examples of penicillinase-resistant penicillins. Methicillin or oxacillin is used to test susceptibil­ity to these anti-staphylococcol penicillins.
•   Amoxicillin and Ampicillin ore equivalent, but amoxicillin is better absorbed after oral administration. These penicillins ore effective agonist penicillin-suscepti­ble organisms and more effective thon penicillin agonist Enterococcus, Listeria, beta-lactamase negative Haemophilus influenzae, Salmonella typhi, Proteus mirovilli and some Escherichia coli. Ampicillin, but not amoxicillin, con be used to treat sensitive gastrointestinal Shigella infection. Amoxicillin is widely used to treat infectious exacerbations of chronic bronchitis but phen-oxymethylpenicillin is pre­ferred for pharyngitis, not least because amoxicillin may Couse o rush in patients with infectious mononucleosis.

• Amoxycillin plus Clavulanate: Combination with clavulanate (o beta-lactamase inhibitor) extends the spectrum of amoxycillin to include otherwise resistant strains of Haemophilus influenzae, Moraxella catarrhalis, some Enterobacteriaceae and most anaerobes including Bacteroides fragilis. However, uncertain CSF penetra­tion by clavulanate precludes the use of this antibiotic in meningitis and brain abscesses.
• Piperacillin: Piperacillin hos the most extensive Gram-negative activity of the pen­icillins. its activity spectrum is that of amoxycillin or ampicillin plus activity against many strains of Pseudomonas aeruginosa. Since they are often synergistic with the aminoglycosides, o combination may be used in life-threatening infections in neu­tropenic patients or patients with proven pseudomonas infection.
•   Piperacillin plus Tazobactam: This combination uses another beta-lactamase inhibitor (tazobactam) which enhances the activity of piperacillin against plasmid mediated (TEM, SHV types) beta-lactamase-producing Gram-negative bacilli such as Klebsiella spp and some E. coli strains. However, this combination is not able to fully overcome the enormous concentrations of chromosomally-mediated beta-lactamase produced by derepressed mutant strains of Enterobacter spp., Citrobacter freundii, Acinetobacter spp., and other Gram-negative bacilli, and therefore combined therapy with an aminoglycoside may be indicated in more serious infections.
• Piperacillin plus Tazobactam: