[The role of coagulase negative staphylococci in mixed infections: association testing as an in vitro model].

Infections due to coagulase negative staphylococci (CNS) are of growing concern mainly in patients hospitalized in intensive care units (ICU). The ability of CNS to adhere and to grow on plastic devices and resistance to many antibiotics, including oxacillin, contributes to their pathogenicity. Using the computer assisted system of the Medical Microbiology Department, the incidences of different pathogens and the coincidence of CNS with other bacteria were evaluated in a surgical department. Staphylococcus aureus revealed to be the predominant pathogen; however, CNS showed an increasing incidence in wound specimens and blood cultures of patients on the ICU. Coincidence of CNS with S. aureus and the nine most frequent species of gram negative bacteria could be shown in 6%. To investigate the influence of beta-lactamases produced by CNS in mixed infections, association experiments were performed. Association means a controlled growth of two or even more bacteria in a susceptibility testing system, either a broth dilution method or an automated broth disk elution method (Cobas Bact). The association experiments showed a significant increase of amoxicillin MIC's of the pathogen associated with CNS. Addition of clavulanic acid restored activity of amoxicillin. It could be shown that in mixed infections CNS may contribute to the failure of antibiotic regimens by production of beta-lactamases.