Selective decontamination of the digestive tract (SDD) is a method for the prevention of infection in the compromised host (Guiot and Van Furth, 1977;Sleyfer et al., 1980). The aim of SDD is the selective elimination of potential pathogenic microorganisms (aerobic gram-negative rods and yeasts) while leaving the anaerobic bacteria of the colon unaffected. This is important, since this anaerobic part of the microflora is responsible for the so-called Colonization Resistance (CR) (Van der Waaij, 1979).
When Staphylococcus aureus is present, cephradine is generally added to the SDD regime. Cephradine is one of the cephalosporines that differs from most antibiotics of this group, because it is well resorbed from the gastro-intestinal tract. When this resorption is complete, cephradine will not affect CR in the colon. Investigations with mice showed that this drug did not affect CR (Thijm and Van der Waaij, 1979).
The dipeptide 13-aspartylglycine ([3-Asp-Gly) is present in the faeces of patients treated with drugs, which do affect CR and it is absent in control persons and patients undergoing SDD treatment (Welling and Groen, 1978;Welling, 1979). When measuring this dipeptide we found that the faeces of some patients undergoing SDD treatment were positive for 13-Asp-Gly whenever cephradine was added.
This prompted us to investigate once more the effect of cephradine on CR. As markers for a disturbed CR we measured [3-Asp-Gly concentrations, since the presence of this compound was shown to be an indicator of a decreased CR. Furthermore, we tried to determine the concentration of cephradine in the faeces by means of high-performance-liquid-chromatography (HPLC).
Four groups of four Swiss mice received orally 30, 70, 140 or 280 rag. kg-1 cephradine respectively. Every three days faeces was sampled. Six rhesus monkeys were treated with SDD by oral administration of nalidixic acid (10 rag-kg-~), polymyxin B (80 rag-kg-1) and nystatin (130 mg. kg-1). After two weeks of this regime cephradine was added in a dose of 90 mg-kg 1. Faeces was sampled .every two or three days.
13-Aspartylglycine was measured as described before (Welling, 1979). From a faeces sample 0.4 g was diluted in 0.6 ml H20. This suspension was centrifuged for 15 min at 15,000 rpm. These supernatants were used for high-voltage paper electrophoresis.
For high-performance liquid chromatographic analyses, 200 gl of the faecal supernatant was mixed with 200 gl TCA (6%) and centrifuged at 15,000 rpm for 15 rain. The supernatant was used for HPLC procedures with 20% methanol in 0.05 sodium phosphate, pH 7.2, as the mobile phase. The column was a reversed phase column: Nucleosil-10 C18 30 x 0.3 cm. The eluate was monitored at 263 nm with a Pye-Unicam detector. The flow rate was 1.0 ml. min-1.
Only in mice treated with the highest dose of cephradine a trace of 13-Asp-Gly was found. In contrast, in the six rhesus monkeys ~-Asp-Gly was measured in the faecal samples in high concentrations from one or two days after starting the cephradine treatment.
In the rhesus monkeys and in the mice treated with the highest dose of cephradine, with HPLC a substantial difference was found between the elution patterns of a faecal supernatant before and during cephradine treatment (see Fig. 1). A number of new peaks, one of which is cephradine, appeared during cephradine treatment. This made it impossible -with the method used -to obtain a base-line separation and thus only approximate cephradine concentrations could be determined: The