Kronvall, G., Ringertz, S., Nystrom, S., Rylander, M. & Theodorsson, E. Comparison of 30 pg and 120 pg gentamicin disks for the prediction of gentamicin resistance in Enterococcus fueculis. APMIS Single strain regression analysis was performed on PDM I1 medium for E. fueculis with 10, 30 and 120 pg gentamicin disks using E. fueculis, strain ATCC 29212 as the reference. This method permits the calculation of zone diameters corresponding to different MIC values for different disk contents. The lack of discrimination between normal low-level resistant strains and high-level resistance using the 10 pg disk was confirmed. However, both the 30 pg and 120 pg disks seemed to provide a separation of the normal low-level gentamicin-resistant population from strains with increased resistance. Since the 30 pg disk is used routinely in some countries, there should be no need for an extra high content disk in these laboratories. This was confirmed when 96 clinical isolates of E. fueculis were analysed and the results of routine disk diffusion tests were compared with the MIC values. Two of the strains showed high-level gentamicin resistance ( > 2000 mgil) and produced no zone of inhibition. The other 94 isolates showed gentamicin MIC values between 4-16 mgil, and 72 of the MIC results were 8 mgil. The zone diameters for these strains ranged between 15 and 25 mm with a mean of 18.2 and a median value of 18 mm. In order to include statistical considerations of the zone size populations for setting of breakpoints, a study of gentamicin zone size distributions was performed for several bacterial species. Inhibition zone diameter values around the 30 pg gentamicin disk for 2079 clinical isolates of E. fueculis, 2268 S. aureus, 3201 E. coli and 547 strains of tl mirabilis from different years were plotted as histograms. Tests for agreement with a Gaussian distribution showed that the histograms were slightly peaked and skewed towards higher zone values. Parametric and nonparametric statistical tests were compared and the results showed that means and medians were very similar and that parametric fractile estimations at the lower end of the histogram populations were conservative and could be used in view of a slightly lower rate of false resistance. The 1% parametric fractile of 12 mm was selected as a suitable breakpoint for the identification of normal, low-level resistant isolates of E. fueculis using the standardized disk test of the Swedish Reference Group for Antibiotics.
Eshali, H., Ringertz, S., Nyström, S. and Faxelius, G. (The Department of Paediatrics, Karolinska Hospital and St. Göran's Children's Hospital and the Department of Clinical Microbiology, Karolinska Hospital, Karolinska Institute, Stockholm, Sweden). Septicaemia with coagulase negative staphylococci in a neonatal intensive care unit. Risk factors for infection, and antimicrobial susceptibility of the bacterial strains. Acta Paediatr Scand Suppl 360: 127, 1989. Septicaemia caused by coagulase negative staphylococci is a problem in the neonatal intensive care units (NICU). The very low birthweight (VLBW) infants are at a special high risk because of their immature host defense. In this study the potential risk factors were compared between the VLBW septicaemia patients and the VLBW infants who had not contracted septicaemia in the NICU. The factor most clearly related with septicaemia was the use of umbilical artery catheters. The strategy against neonatal septicaemia includes restriction of the use of intravascular catheters. Also enhancement of the host defense by immunoglobulin therapy is considered. The coagulase negative staphylococci were multiresistant to antibiotics. The combination of netilmicin and benzylpenicillin covered the bacteria found in septicaemia cases in the NICU, and is now the standard treatment in suspected cases. Coagulase negative staphylococci are treated with vancomycin or netilmicin.
The efficacy of the antimicrobial removal device in facilitating isolation of bacteria from blood containing antibiotics was evaluated in a multicenter study. Blood specimens from 143 patients who had culture-proven septicemia and had been on antibiotic treatment for at least 24 hours were recultured with and without the aid of the device. Septicemia persisted in 33 of the patients and use of the device improved the isolation rates: 63% of the gram-positive bacteria and 32% of the gram-negative bacteria were isolated only with the aid of the device. Use of the device was an advantage in 10 of 13 cases where the antibiotic concentration in the bottles exceeded the MIC for the infecting microorganism but in only 5 of 19 cases where the concentration was lower than the MIC. It is concluded that the antimicrobial removal device may be useful in patients already on antibiotic treatment, but that its use is limited by the cost and the amount of work involved.
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