Emergence of resistance under SDD includes resistance to the systemic antibiotic used in SDD, resistance to topical antibiotics, and the selection of indigenous aerobic Gram-positive cocci.
In general, resistance to systemic antibiotics develops in the oropharynx and gut, where high concentrations of potentially pathogenic micro-organisms are present and subinhibitory concentrations of antibiotics secreted via saliva or bile are reached. Superinfections are mostly secondary endogenous infections with resistant potentially pathogenic micro-organisms selected in the oral or intestinal flora. During SDD the oral cavity and gut are decontaminated with very high concentrations of topical antibiotics, preventing selection of resistance against the systemic antibiotics.
Acquired resistance of Gram-negative potentially pathogenic micro-organisms to polymyxin has never been described, although some strains (e.g. Proteus species) are intrinsically insensitive to polymyxin. Resistance to topical tobramycin should be related to the levels of tobramycin in the gut, being more than 100 times higher than the maximum attainable blood levels. Therefore strains considered to be resistant to tobramycin according to the standard criteria can often be eliminated by topical tobramycin. Emergence of resistance should not be analyzed by comparing the percentages of resistant isolates. It is important to exclude the copy strains (i.e. the same micro-organism from the same site from the same patient cultured more than once) and to compare the incidence of colonization/infection by resistant strains. Eliminating the sensitive Gram-negative potentially pathogenic micro-organisms with SDD results in a higher percentage of resistant isolates than is found without SDD, where the resistant isolates are diluted by the large number of sensitive potentially pathogenic micro-organisms. This may lead to the erroneous conclusion that emergence of resistance may be promoted by SDD. In most controlled SDD trials no evidence for emergence of resistance has been found. In many ICUs where the same antibiotic regimen for SDD has been used for more than 10 years there is still no evidence of emergence of resistance.
It has been suggested that SDD may lead to a Gram-positive selection. During successful decontamination, enterococci and Staph. epidermidis, which belong to the normal indigenous flora and are intrinsically insensitive to the topical antibiotics, are the only micro-organisms isolated. Controlled trials that specifically studied enterococci and Staph. epidermidis did not show increased colonization or infection with these micro-organisms during SDD.
MRSA is an inherent limitation of the PTA regimen. In an environment where MRSA is endemic, SDD may select this micro-organism. In this situation SDD should be reconsidered or vancomycin should be added to the PTA regimen.
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