Mupirocin is active against Gram-positive cocci, including methicillin-susceptible and methicillin-resistant strains of Staphylococcus aureus. Mupirocin inhibits staphylococcal isoleucyl tRNA synthetase. Low-level resistance, defined as a minimum inhibitory concentration (MIC) of up to 100 mcg/mL, is due to point mutation in the gene of the target enzyme. Low-level resistance has been observed after prolonged use. However, local concentrations achieved with topical application are well above this MIC, and this level of resistance does not lead to clinical failure. High-level resistance, with MICs exceeding 1000 mcg/mL, is due to the presence of a second isoleucyl tRNA synthetase gene, which is plasmid-encoded. High-level resistance results in complete loss of activity. Strains with high-level resistance have caused hospital-associated outbreaks of staphylococcal infection and colonization. Although higher rates of resistance are encountered with extensive use of mupirocin, most staphylococcal isolates are still susceptible.
Mupirocin is indicated for topical treatment of minor skin infections, such as impetigo (see Chapter 61). Topical application over large open areas, such as pressure ulcers or surgical wounds, is an important factor leading to emergence of mupirocin-resistant strains and is not recommended. Mupirocin temporarily eliminates S aureus nasal carriage by patients or health care workers, but results are mixed with respect to its ability to prevent subsequent staphylococcal infection. Patients most likely to benefit from decolonization are those undergoing orthopedic or cardiothoracic procedures.
The polymyxins are a group of basic peptides active against Gram-negative bacteria and include polymyxin B and polymyxin E (colistin). Polymyxins act as cationic detergents. They attach to and disrupt bacterial cell membranes. They also bind and inactivate endotoxin. Gram-positive organisms, Proteus sp, and Neisseria sp are resistant.
Owing to their significant toxicity with systemic administration (especially nephrotoxicity), polymyxins were, until recently, largely restricted to topical use. Ointments containing polymyxin B, 5000 units/g, in mixtures with bacitracin or neomycin (or both) are commonly applied to infected superficial skin lesions. Emergence of strains of Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacteriaceae that are resistant to all other agents has renewed interest in polymyxins as parenteral agents for salvage therapy of infections caused by these organisms.
Urinary antiseptics are oral agents that exert antibacterial activity in the urine but have little or no systemic antibacterial effect. Their usefulness is limited to lower urinary tract infections.
At therapeutic doses, nitrofurantoin is bactericidal for many Gram-positive and Gram-negative bacteria; however, P aeruginosa and many strains of Proteus are inherently resistant. Nitrofurantoin has a complex mechanism of action that is not fully understood. Antibacterial activity appears to correlate with rapid intracellular conversion of nitrofurantoin to highly reactive intermediates by bacterial reductases. These intermediates react non-specifically with many ribosomal proteins and disrupt metabolic processes and the synthesis of proteins, RNA, and DNA. It is not known which of the multiple actions of nitrofurantoin is primarily responsible for its bactericidal activity.
There is no cross-resistance between nitrofurantoin and other antimicrobial agents, and resistance emerges slowly. As resistance to trimethoprim-sulfamethoxazole and fluoroquinolones has become more common in Escherichia coli, nitrofurantoin has become an important alternative oral agent for treatment of uncomplicated urinary tract infection.