Trimethoprim used for selective decontamination of the digestive tract in rats: possible route of excretion

The effect of different oral antibiotics on the gastrointestinal microflora of a wild rodent (Aethomys namaquensis)

Gut sterilization via the oral administration of antibiotics facilitates physiological studies of the nutritionally important relationship between intestinal microflora and the host. However, the composition of gut flora is extremely variable, and as a result, the efficacy of antibiotics in achieving gut sterilization varies considerably between species. We tested the effectiveness of three antibiotic cocktails in sterilizing the gut of a rodent pollinator, the Namaqua rock mouse (Aethomys namaquensis). The cocktails were (1) streptomycin sulfate and bacitracin (previously used with domestic mice and rats), (2) chloramphenicol and bacitracin (based on antibiotic screening tests performed on faecal flora) and (3) Baytril 10% oral solution (a veterinary antimicrobial agent containing enrofloxacin). We tested for antibiotic inactivation by determining bacterial viability through fluorescence staining of faecal samples. We also tested techniques to maintain sterility during antibiotic treatment without the benefit of a laminar flow cabinet. Antibiotics were administered orally in food and water consumed ad libitum over 4 consecutive days. Antibiotic effectiveness was assessed by culturing anaerobic bacteria from faecal samples collected before and after each antibiotic treatment. Treatment with Baytril 10% oral solution eliminated or significantly reduced faecal flora, whereas other antibiotics did not. This study clearly demonstrates the importance of testing the effectiveness of antibiotics before their use in studies that involve antibiotic-treated subjects, particularly if these are species previously untested.

History of recognition and measurement of colonization resistance of the digestive tract as an introduction to selective gastrointestinal decontamination

Selective decontamination of the digestive tract was developed following the observation that the digestive tract normally has considerable resistance to colonization by newly ingested bacteria. The research that eventually led to selective decontamination was performed because in the late 1960s and early 1970s, the need for prophylaxis against Gram-negative infections in immuno-compromized patients became evident. At that time, the relatively small number of antibiotics available for therapy of serious infections often lead to treatment failure. To introduce the subject of selective decontamination, this paper, therefore, starts with a short historical overview of the kind of infectious agents as well as the antibiotics available in the 1960s; particularly regarding the type and treatment of infections in severely compromised patients. The fact that the possibilities in infectious diseases were limited was the reason for our experimental search for ways of successful prophylactic treatment with minimal risk of development of resistance.

Clinical use of selective decontamination: the concept

Infections can be classified according to: (1) the type of offending microorganism (virus, bacteria, fungi, parasites), (2) according to the clearance by the defence system (T cell dependent/independent) and (3) in case bacteria are the causative agents in Gram-positive and Gram-negative infections. The latter classification in Gram-positive and Gram-negative infections has appeared to have a practical consequence. Gram-negative bacteria, often involved in major infections and yeasts, appear to play practically no role in the intestinal ecological system. Consequently, it is nowadays increasingly attempted to eliminate Gram-negative bacteria and yeasts selectively from the digestive tract with antimicrobial agents. Selective suppression of Gram-positive bacteria may severely affect the ecosystem of the digestive tract. This selective suppression of Gram-negatives must be continued as long as patients are immunocompromised (locally or systemically) and is called selective decontamination of the digestive tract.

A norfloxacin dose finding study for selective decontamination of the digestive tract in pigs

Five pigs were treated orally with norfloxacin for 5 consecutive days in two well-separated periods. This was done to determine the lowest dose required to free the pigs of Enterobacteriaceae. In the first period of the study, the animals were treated with 400 mg per day while in the second treatment 800 mg norfloxacin was given. Daily faecal culturing indicated that the faeces became free of Enterobacteriaceae in 3-5 days when treated with 400 mg/day, while all animals were found negative on culturing after 3 days of treatment with 800 mg/day. Investigation of the concentration of norfloxacin in the faeces revealed that a substantial fraction of the dose was either absorbed or inactivated by faecal substances. An in vitro study in faeces confirmed that a substantial part, some 75% of the dose, may have been inactivated by intestinal contents. This finding helps to explain the much lower concentration of 120 mg norfloxacin per kg of faeces in the pig in comparison with the almost tenfold higher concentration reported to develop in man during treatment with an identical dose.