Antibiotics for Gram-Positive Infections

Review and outline the four broad mechanisms by which bacteria can be resistant to antibiotics.
Explain the mechanism of resistance of MRSA to beta-lactam antibiotics.

• Introduction to Antibiotics
• Antibiotic Resistance

Explain why 80% or more of S. aureus strains are resistant to penicillin.

Roughly 80% of strains of S. aureus are able to produce penicillinases (enzymes that break down penicillin), resulting in them being resistant to penicillin.

Name 2 penicillins and two cephalosporins that are resistant to staphylococcal beta lactamase.

• Penicillins: Dicloxacillin, flucloxacillin, methicillin, nafcillin
• Cephalosporins: Ceftazidime, cefepime. Other good ones against penicillinases include cephalexin and cephalothin.

Discuss the use of and mechanism of beta-lactamase inhibitors and name an oral and parenteral drug combination in which these are used.

Beta-lactamase inhibitors can be given in combination with beta-lactam antibiotics in order to prevent bacterial beta-lactamases from breaking down the antibiotic. Examples of oral drug combinations are amoxicillin + clavulanic acid and trimethoprim + sulphamethoxazole. An example of a parenteral drug combination is piperacillin + tazobactam.

Name 4 oral antibiotics, each from a different class, which may have activity against MRSA.

• Cephalosporins: Ceftaroline (the only beta-lactam that works against MRSA)
• Macrolides: Erythromycin
• Lincosamides: Clindamycin
• Sulfonamides: Trimethoprim + sulfamethoxazole (co-trimoxazole)
• Fluoroquinolones: Ciprofloxacin

Name an IV antibiotic used to treat serious MRSA infections.

Vancomycin (a glycopeptide). Some oral antibiotics, such as rifampicin, can also be given via IV for serious infections.

Name an infection where formal MIC testing is done on the causative organism and discuss why.

Formal MIC testing is often done on Viridans streptococci as they have highly variable degrees of susceptibility to penicillin.

Explain why a 10 day course of antibiotics is used in the treatment of streptococcal pharyngitis. Name two drugs used for this purpose.

Penicillin is usually used to treat streptococcal pharyngitis, but azithromycin can be used if the patient is allergic. Antibiotics are usually given for 10 days in order to protect against development of rheumatic fever (see here for more information about rheumatic fever).