<prompt> Streptococcus pyogenes have not shown resistance to:PenicillinErythromycinTetracyclineCotrimoxazole  <response>: Ans: a Penicillin Ref:KDT, 6th ed, p. 696Streptococci except viridians, group D or enterococci are highly sensitive to penicillin.Enterococci are usually treated with penicillinampicillin gentamicin combination.Patients allergic or resistant to penicillin should be treated with vancomycin gentamicin.Penicillins:They are a lactam antibiotics which interfere with synthesis of bacterial cell wall. They inhibit the transpeptidases so that cross linking in the bacterial cell wall does not take place.The transpeptidases and the related proteins constitute the penicillin binding proteins PBPs.Resistance to penicillins and other b-lactams is due to one of four general mechanisms:1 Inactivation of antibiotic by b-lactamase2 Modification of target PBPs3 Impaired penetration of drug to target PBPs4 Efflux. b-Lactamase production is the most common mechanism of resistance. Altered target PBPs penicillin binding proteins are the basis of methicillin resistance in staphylococci and of penicillin resistance in pneumococci and enterococci. Resistance due to impaired penetration of antibiotic to target PBPs occurs only in gram-negative species because of their impermeable outer cell wall membrane, which is absent in gram-positive bacteria.Classification1. Acid resistant alternative to penicillin G: Phenoxymethyl penicillin Penicillin V2. Penicillinase resistant penicillins: Methicillin, cloxacillin3. Extended spectrum pehicillin:a Amino penicillin: Ampicillin, bacampicillin, amoxicillinb Carboxypenicillin: Carbenicillin, ticarcillinc Ureidopenicillin: Piperacillin, mezlocillinBeta-lactamase inhibitors: Clavulanic acid, sulbactum, tazobactum.Penicillin G is the drug of choice for Group A B streptococcal infections, syphilis, anthrax, actinimycosis, tetanus, clostridial myonecrosis, rat-bite fever and Pasteurella multocida infections. 