Drug Therapy and Impaired Host Defense Mechanisms Antibiotics alter the normal microflora of the skin, mucous membranes, and GI tract and may result in colonization by new organisms. Colonization is harmless unless followed by superinfection, which refers to invasion by indigenous or environmental organisms resistant to the administered antibiotic. Factors predisposing to superinfection include extremes of age, debilitating diseases, and prolonged treatment with antibiotics, especially broad-spectrum ones. Superinfections usually appear on the 4th or 5th day of therapy and may convert a benign, self-limited disease into a serious, prolonged, or even fatal one. The diagnosis of superinfection by a normally commensal organism is certain only when the organism is recovered from blood, CSF, or body cavity fluid.
Most opportunistic infections result from the leukopenia.
Corticosteroids may reactivate quiescent pulmonary TB, histoplasmosis, coccidioidomycosis, and blastomycosis. They are commonly due to Staphylococcus, Enterobacter, Klebsiella, Serratia, Pseudomonas, Proteus, Acinetobacter, Aspergillus, or Candida. Gram-negative bacteria, particularly Pseudomonas and Serratia, and other multiply resistant organisms, alone or in combination with staphylococci, cause soft tissue infections and bacteremia in severely burned patients. Significant bacteriuria develops in patients with indwelling urethral catheters, increasing the risk of cystitis, pyelonephritis, and bacteremia with gram-negative bacilli. Sepsis from IV catheter sites, due to staphylococci, gram-negative bacilli, or Candida, may cause local suppuration or severe and sometimes fatal systemic infections.