Molecular basis of chloramphenicol and thiamphenicol toxicity to DNA in vitro

Impact of dietary-nucleotides and Saccharomyces cerevisiae-derivatives on growth-performance, antioxidant-capacity, immune-response, small-intestine histomorphometry, caecal-Clostridia, and litter-hygiene of broiler-chickens treated with florfenicol

Stress in poultry production is energy-demanding. Nucleotides and yeast cell-wall products are essential nutrients for broiler performance, gut function, and immune response. Antibiotics, like florfenicol, negatively affect the immune system. A total of 600 one-d-old broiler chickens (Cobb-500) were weighed and randomly allotted into four groups with three replicates each. The control group (G1) received the basal diet, G2 received a diet supplemented with a combination of nucleotides and Saccharomyces cerevisiae derivatives (250 g/Ton), G3 received the basal diet and medicated with florfenicol (25 mg/Kg body weight) in drinking water for 5 days, while G4 received a combination of nucleotides and Saccharomyces cerevisiae-derivatives (250 g/Ton) and medicated with florfenicol in drinking water. Growth performance criteria were recorded weekly. Blood, intestinal contents, small-intestine sections, and litter samples were collected to measure birds' performance, carcass yields, leukocytic counts, antioxidant capacity, antibody titres, phagocytic index, caecal Clostridia, intestinal histomorphometry, and litter hygiene. Nucleotide-supplemented groups (G2 and G4) revealed significant (p ≤ 0.05) improvements in feed conversion, and body weight, but not for carcass yields in comparison to the control. Dietary nucleotides in G2 elevated blood total proteins, leucocytic count, antioxidant capacity, and phagocytic index, while they lowered blood lipids and litter moisture and nitrogen (p ≤ 0.05). Dietary nucleotides in G4 ameliorated the immunosuppressive effect of florfenicol (p ≤ 0.05) indicated in reducing caecal Clostridia, improving duodenal and ileal villi length, and increasing blood albumin and globulin levels, and phagocytosis%. Supplementing diets with nucleotides and yeast products has improved the immune system and provided a healthier gut for broilers.

Florfenicol induces oxidative stress and hepatocyte apoptosis in broilers via Nrf2 pathway

In order to explore the mechanism of liver injury induced by florfenicol (FFC) in broilers, one hundred and twenty broilers were randomly divided into six groups, twenty broilers in each group. Except for control group, the other five groups were given different doses of FFC (0.15 g/L, 0.3 g/L, 0.6 g/L, 1.2 g/L and 1.8 g/L) in drinking water. After five days of continuous use, blood was collected from the subpterional vein and the chickens' liver were obtained. Chicken weight gain and liver indices were calculated; blood routine analysis was performed; the oxidative stress and apoptosis of hepatocytes was detected. The results showed that compared with the control group, except for 0.15 g/L FFC, the other doses of FFC significantly decreased the weight gain, white blood cell (WBC) and platelet (PLT) contents in blood, 0.3 g/mL FFC and 1.8 g/L FFC significantly reduced the content of hemoglobin (RGB) (P < 0.05); all doses of FFC significant decreased red blood cell (RBC) increased Alanine aminotransferase (ALT) and Aspartate aminotransferase (AST) contents in serum of chickens (P < 0.05), and significantly decreased the contents of albumin (ALB) and total protein (TP) in serum (P < 0.05), but had no significant effect on alkaline phosphatase (ALP) contents(P > 0.05). FFC significantly increased malondialdehyde (MDA) content in serum and liver tissues, but decreased glutathione (GSH), Superoxide dismutase (SOD) and catalase (CAT) content (P < 0.05), and significantly inhibited the mRNA transcription and protein expression of antioxidant proteins nuclear factor-erythroid 2-related factor 2 (Nrf2), heme oxygenase-1 (HO-1) and NAD(P)H dehydrogenase quinone-1 (NQO-1)(P < 0.05). FFC also inhibited the content and the transcription level of cytochrome P4501A1(CYP1A1) and CYP2H1 in liver (P < 0.05). At the same time, FFC significantly promoted the apoptotic rate of hepatocytes and the mRNA transcription and protein expression of caspase-3 and caspase-6 (P < 0.05). With the increase of FFC concentration, liver injury became more and more serious, which affected liver function in chickens by inhibiting enzyme activity in Nrf2-ARE pathway to increase oxidative stress and promoting apoptotic protein expression to accelerate hepatocyte apoptosis.

Similar Gastro-Intestinal Exposure to Florfenicol After Oral or Intramuscular Administration in Pigs, Leading to Resistance Selection in Commensal Escherichia coli

Florfenicol, which is licensed for veterinary use only, proves to be a potent antimicrobial for treatment of respiratory disease. However, the subsequent exposure of the gut microbiota to florfenicol is not well described. Hence, the effect of various administration protocols on both plasma and gastro-intestinal florfenicol concentrations in pigs was evaluated. In field situations were simulated by application of different administration routes and dosages [single oral bolus at 10 or 5 mg/kg body weight (BW), medicated feed at 10 or 5 mg/kg BW and intramuscular injections at 15 or 30 mg/kg BW]. After intramuscular administration of 30 mg florfenicol/kg BW, gastro-intestinal concentrations of florfenicol, quantified 10 h after the last administration, were significantly elevated in comparison with the other treatment groups and ranging between 31.5 and 285.8 μg/g over the different gut segments. For the other treatment groups, the influence of dose and administration route was not significantly different. Bacteriological analysis of the fecal samples from the animals at the start of the experiment, demonstrated the presence of both florfenicol susceptible (with minimal inhibitory concentration (MIC) values of 2-16 μg/mL) and florfenicol resistant (MIC ≥ 256 μg/mL) Escherichia coli isolates in all treatment groups. Following, at 10 h after the last administration the susceptible E. coli population was eradicated in all treatment groups due to the high intestinal florfenicol concentrations measured. Moreover, selection of the resistant E. coli strains during treatment occurred in all groups. This is likely related to the fact that the different treatment strategies led to high gastro-intestinal concentrations albeit not reaching the high magnitude of MIC values associated with florfenicol resistance (≥256 μg/mL). Conclusively, in our experimental setup the administration route and dose alterations studied, had no influence on monitored florfenicol resistance selection in E. coli from the microbiota.

Assessment of florfenicol as a possible treatment for chlamydiosis in koalas (Phascolarctos cinereus)

OBJECTIVE

Because of limited availability of chloramphenicol to veterinary suppliers, a preliminary study was performed to predict whether an analogue, florfenicol, is an efficacious treatment for chlamydiosis in koalas.

METHODS

Florfenicol was administered to koalas with naturally occurring chlamydiosis at 20 mg/kg SC (n = 3) and at 5 mg/kg (n = 3) and 10 mg/kg (n = 3) IV. The estimated areas under the plasma concentration versus time curves (AUC) were compared with the minimum inhibitory concentration to inhibit Chlamydia pecorum. Clinical data were also examined from field trials conducted on koalas (n = 19) with naturally occurring chlamydiosis and treated with florfenicol at a range of dosages (5-20 mg/kg SC and 6-15 mg/kg IV). Florfenicol binding to proteins in plasma was also determined.

RESULTS

Florfenicol was not detectable in plasma 24 h post-administration at 20 mg/kg SC. The estimated AUC_0-24 h following administration at 10 mg/kg IV suggests florfenicol might be effective against Chlamydia spp. via this route. Florfenicol binding to plasma proteins was 13.0% (± 0.30 SEM). After treatment with florfenicol in field trials, 5 of 19 koalas (26%) were released without further treatment, 4 with no long-term follow-up; 6 (32%) required additional treatment with chloramphenicol to resolve chlamydiosis; 7 (36%) failed to clinically improve, of which 3 had clinical signs and/or necropsy findings suggestive of antibiotic-related gastrointestinal dysbiosis; another koala died within minutes of florfenicol administered IV at 7 mg/kg.

CONCLUSION

When administered at dosages tolerable in the field, florfenicol is a problematic treatment for chlamydiosis based on equivocal outcomes and plasma concentrations below those that inhibit the pathogen.

Toxicity to the hematopoietic and lymphoid organs of piglets treated with a therapeutic dose of florfenicol

Florfenicol (FLO) is a broad-spectrum antibacterial agent for treatment of bacteriosis of piglets in veterinary practice. To study the toxicity to the hematopoietic and lymphoid organs of piglets treated with a therapeutic dose of FLO, 20 healthy weaned piglets were selected and randomly divided into two groups. Piglets in the FLO group were fed with fodder supplemented with 30mg/kg BW of FLO twice a day for 10 days. Blood samples were drawn at four time points: 1 day before FLO administration and 1, 7, and 14 days post-withdrawal. Three or four piglets were euthanized at each time point post-withdrawal and tissue samples (bone marrow, thymus and spleen) were collected for fixation and cryostorage. The levels of classical swine fever virus (CSFV) antibody against the vaccine, the concentrations of Hsp70 and IL-6 in serum and Hsp70 in tissues, and the mRNA expression levels of B-cell lymphoma 2 (bcl-2) and tumor suppressor p53 were detected, the hematology of the piglets were analyzed, and the histopathology and the status of apoptosis of the hematopoietic and lymphoid organs was examined. The results showed changes in several indicators in the FLO group 1 day post-withdrawal: the concentration of red blood cells (RBCs) was decreased, and that of platelets (PLTs) was significantly lower (p<0.05); the volumes of RBC and PLT were increased; the sum of blood lymphocytes was statistically decreased (p<0.05); the concentration of IL-6 was significantly increased (p<0.05); the concentrations of Hsp70 in serum and tissues were increased; obvious atrophy of the hematopoietic cell lines and partial replacement by fat cells were observed in bone marrow; thymus and spleen tissues showed lower concentrations and sparser arrangement of lymphocytes in the thymic medulla and white pulp of the spleen respectively; and the mRNA expression levels of bcl-2 in the three tissues were up-regulated, while that of p53 was down-regulated. With time after cessation of FLO administration, the indicators of the FLO group gradually returned to close to that of the control group and the histological lesions of the tissues gradually recovered, and the differences in the densities of lymphocytes and cell arrangements in the tissues between two groups gradually decreased. In conclusion, a therapeutic dose of FLO induces temporary toxicity in the hematopoietic and lymphoid organs of piglets to some extent, and influences hemopoiesis and immune function. These effects gradually decrease after cessation of FLO administration.

Adverse effects of antimicrobials via predictable or idiosyncratic inhibition of host mitochondrial components

This minireview explores mitochondria as a site for antibiotic-host interactions that lead to pathophysiologic responses manifested as nonantibacterial side effects. Mitochondrion-based side effects are possibly related to the notion that these organelles are archaic bacterial ancestors or commandeered remnants that have co-evolved in eukaryotic cells; thus, this minireview focuses on mitochondrial damage that may be analogous to the antibacterial effects of the drugs. Special attention is devoted to aminoglycosides, chloramphenicol, and fluoroquinolones and their respective single side effects related to mitochondrial disturbances. Linezolid/oxazolidinone multisystemic toxicity is also discussed. Aminoglycosides and oxazolidinones are inhibitors of bacterial ribosomes, and some of their side effects appear to be based on direct inhibition of mitochondrial ribosomes. Chloramphenicol and fluoroquinolones target bacterial ribosomes and gyrases/topoisomerases, respectively, both of which are present in mitochondria. However, the side effects of chloramphenicol and the fluoroquinolones appear to be based on idiosyncratic damage to host mitochondria. Nonetheless, it appears that mitochondrion-associated side effects are a potential aspect of antibiotics whose targets are shared by prokaryotes and mitochondria-an important consideration for future drug design.

The structural basis for substrate versatility of chloramphenicol acetyltransferase CATI

Novel antibiotics are needed to overcome the challenge of continually evolving bacterial resistance. This has led to a renewed interest in mechanistic studies of once popular antibiotics like chloramphenicol (CAM). Chloramphenicol acetyltransferases (CATs) are enzymes that covalently modify CAM, rendering it inactive against its target, the ribosome, and thereby causing resistance to CAM. Of the three major types of CAT (CAT(I-III)), the CAM-specific CAT(III) has been studied extensively. Much less is known about another clinically important type, CAT(I). In addition to inactivating CAM and unlike CAT(III), CAT(I) confers resistance to a structurally distinct antibiotic, fusidic acid. The origin of the broader substrate specificity of CAT(I) has not been fully elucidated. To understand the substrate binding features of CAT(I), its crystal structures in the unbound (apo) and CAM-bound forms were determined. The analysis of these and previously determined CAT(I)-FA and CAT(III)-CAM structures revealed interactions responsible for CAT(I) binding to its substrates and clarified the broader substrate preference of CAT(I) compared to that of CAT(III).

Evaluation of antibiotic susceptibilities of Ehrlichia canis, Ehrlichia chaffeensis, and Anaplasma phagocytophilum by real-time PCR

We determined MICs of antibiotics against Anaplasma phagocytophilum, Ehrlichia chaffeensis, and Ehrlichia canis by real-time quantitative PCR. The doubling times of the organisms were established: 19 h for E. chaffeensis, 26 h for A. phagocytophilum, and 28 h for E. canis. In comparison to the reference method for determining sensitivities, which uses Diff-Quick staining, our PCR assay was very sensitive and specific. We confirmed that doxycycline and rifampin are highly active against these bacteria and found variable susceptibilities to fluoroquinolones; A. phagocytophilum was susceptible, but E. canis and E. chaffeensis were only partly susceptible. Beta-lactam compounds, cotrimoxazole, macrolide compounds, and telithromycin showed no activity against any of the three organisms. Thiamphenicol was found to be more active than chloramphenicol. For the first time, we showed that these three species have numerous point mutations in their 23S RNA genes, with those at positions 754, 2057, 2058, 2059, and 2611 (Escherichia coli numbering) known to confer resistance to macrolide compounds in other bacteria. The role of each of these mutations in resistance to these drugs should be investigated in the future. Our study confirms previous reports that quantitative PCR is a reliable method for determining antibiotic susceptibility; therefore, it might be useful for screening new drugs.

Development of a rapid and sensitive SPE-LC-ESI MS/MS method for the determination of chloramphenicol in seafood

A method based on liquid chromatography-tandem mass spectrometry was developed and validated for the qualitative and quantitative detection of chloramphenicol (CAP) in seafood samples. The analysis of CAP residues in seafood is important because CAP can cause serious acute reactions in humans, including aplastic anemia and leukemia. The proposed methodology includes a cleanup solid-phase extraction procedure with high recovery efficiency (>90%). Chromatographic separation of CAP and the internal standard (IS) was carried out on a C(18) column, followed by mass spectrometric detection using electrospray ionization in the negative-ion mode. The precursor/product ion transitions 321-->257 (CAP) and 354-->290 (IS) were monitored. Statistical evaluation of this multiple reaction monitoring mass spectrometric procedure reveals good linearity, accuracy, and inter- and intraday precisions. The limit of detection was 0.1 ng/mL, and the limit of quantification for CAP in seafood samples is 0.02 microg/kg. Application in seafood samples allowed the detection of CAP in low parts per billion levels.

Lack of carcinogenicity of thiamphenicol in F344 rats

The carcinogenicity of thiamphenicol (TAP), an antibiotic drug, was examined in Fischer 344 rats of both sexes. TAP was given ad lib. in drinking water at levels of 0 (control), 125 or 250 ppm to groups of 50 male and 50 female rats for 2 yr. The treatment did not affect mortality and no toxic lesions were specifically induced. Furthermore, the incidence of tumours in the treated groups did not show any significant dose-related increase as compared with the control group. The results thus indicate that TAP is neither toxic nor carcinogenic, for any organs or tissues of F344 rats when given continuously at levels of 125 or 250 ppm in drinking water for 2 yr.

Reductive activation of nitroheterocyclic compounds

1. Nitroaromatic compounds are important chemotherapy agents. 2. Their selective toxicity is determined by reduction to the biologically active form in the absence of oxygen. 3. Nitroaromatics are extensively used in the treatment of anaerobic infections and to target hypoxic tumor cells in cancer therapy. 4. Possible mutagenic action is related to the relative ease of nitro group reduction. 5. The mode of action and clinical application of nitroaromatic compounds is summarized.

Posttranscriptional repression of Escherichia coli OmpF protein in response to redox stress: positive control of the micF antisense RNA by the soxRS locus

The soxRS regulon is a cornerstone of the adaptive defense systems of Escherichia coli against oxidative stress. Unexpectedly, activation of this regulon also enhances bacterial resistance to multiple antibiotics that seem unrelated to oxygen radicals. We previously correlated this multiple antibiotic resistance with a reduced rate of synthesis of the OmpF outer membrane porin that does not affect the OmpC or OmpA porins. Studies presented here, with operon and gene fusions of ompF to lacZ, show that the soxRS-dependent repression of OmpF is achieved posttranscriptionally. We also show posttranscriptional repression of OmpF mediated by the soxQ1 mutation, which maps to the marA locus. These repressions are dependent on the micF gene, which encodes a small RNA partially complementary to the 5' end of the ompF message. Northern (RNA) blotting experiments show that micF transcription is strongly inducible by the superoxide-generating agent paraquat in a manner that depends completely on the soxRS locus. The soxR-constitutive and soxQ1 mutations elevate the expression of micF in the absence of redox stress. However, the antibiotic resistance mediated by a soxR-constitutive mutation was only partially reversed upon deletion of micF. The soxRS regulon therefore includes other components that contribute to general antibiotic resistance, although the relation of this phenotype to oxidative stress remains to be established.

Therapy of bovine bacterial pneumonia

Practical strategies for developing rational therapeutic regimens based on in vitro sensitivity and pharmacokinetic disposition are presented. Special attention is given to Pasteurella haemolytica, which is regarded as the most frequent cause of bovine bacterial pneumonia. Bacterial-dependent and host-dependent causes of therapeutic failure and potentially valuable novel therapies and drug combinations are considered.

Interaction of drugs with extranuclear genetic elements and its consequences

Bacterial ancestry of mitochondria and plastids is now generally accepted. Both organelles contain their own DNA and transcription-translation apparatus of a prokaryotic type. Due to this fact these systems carry bacteria-like properties. Thus organellar DNA and ribosomes are essentially different from nuclear DNA and cytoplasmic ribosomes in physical as well as in functional respects. Due to the bacterial character of both types of organelles they are susceptible to various antibacterial chemicals. Inhibitors of bacterial protein synthesis inhibit mitochondrial (plastidial) biogenesis. Therefore the cellular content of mitochondria (plastids)-made proteins decreases during cytoplasmic turnover or cell division in the presence of these drugs. Such drug activity consequently leads to a reduced capacity for oxidative phosphorylation or photosynthesis. Organellar genomes are less stable and more sensitive to mutagenesis as compared to nuclear genome. It means also that genotoxic agents induce various disorders of mitochondrial (plastidial) functions. Impairments in the respiratory chain are associated with structural as well as functional abnormalities of mitochondria. These are clinically expressed mostly in tissues with a high demand for ATP: brain, heart, skeletal muscle, and retina. On the other hand, some antibacterial inhibitors of mitochondrial biogenesis (e.g., tetracyclines) inhibit selectively tumor cell proliferation. Therefore they may be considered for use in anticancer therapy. The article summarizes the response of mitochondria and plastids in various organisms to drugs and environmental xenobiotics. Various model organisms suitable for detection of xenobiotic effect on mitochondria (plastids) are presented as well as the possible consequences of such interaction.

Electrochemical characteristics of nitro-heterocyclic compounds of biological interest. II. Nitrosochloramphenicol

The electrochemical characteristics of nitrosochloramphenicol have been studied in aqueous buffer systems (pH 7.1) using direct current (d.c.) and differential pulse polarography, cyclic voltammetry and coulometric techniques. Up to 4 charge-transfer steps can be identified. The first reduction step is reversible both chemically and electrochemically, the charge-transfer product showing no tendency to undergo further reaction on the electrochemical time-scale. In contrast, the second reduction step is irreversible, with the product undergoing a fast following reaction to yield a redox-active species which was detected by cyclic voltammetry. From the data and by comparison with related systems, two reduction mechanisms are possible and are discussed.

Chloramphenicol: magic bullet or double-edge sword?

The genetic and genotoxic potentials of chloramphenicol are reviewed and analyzed. Although this widely used antimicrobial agent appears to cause chromosomal effects in somatic cells, in view of the consistent absence of other genetic effects, these cytogenetic abnormalities are ascribed to non-genotoxic causes. It is pointed out that despite its widespread use in human medicine, chloramphenicol has not been systematically tested for genotoxicity.

DNA damage in intact cells induced by bacterial metabolites of chloramphenicol

Four chloramphenicol (CAP) metabolites known to be produced by intestinal bacteria were examined with respect to their capacity to induce DNA damage in intact cells. The induction of DNA single-strand breaks in Raji cells, activated human lymphocytes, and human marrow cells was assayed by the alkaline elution technique. One of the four compounds tested, dehydro-CAP, was capable of inducing DNA single-strand breaks in all three cell systems at concentrations of 10(-4) M. This effect is comparable to that observed previously with nitroso-CAP, the nitroreduction intermediate of CAP. The nitroreduction of dehydro-CAP by human bone marrow cell homogenate was detected by the production of the corresponding amino derivative amounting to 5.6 X 10(-5) M from 2 X 10(-3) M substrate under aerobic conditions. In sharp contrast, nitroreduction of CAP by bone marrow could not be demonstrated. The genotoxicity of dehydro-CAP, its relative stability compared to the nitroso-CAP, and its nitroreducibility by bone marrow suggest that this bacterial metabolite of CAP may play a key role as a mediator of aplastic anemia in the predisposed host.

Florfenicol in non-lactating dairy cows: pharmacokinetics, binding to plasma proteins, and effects on phagocytosis by blood neutrophils

Serial blood samples were collected and plasma concentrations of florfenicol (FLO) were measured following the administration of an intravenous bolus of 50 mg/kg FLO to five healthy non-lactating dairy cows. A triexponential equation provided the best fit of the data for four of the five cows. The mean value for beta corresponded to a half-life of 3.2 h. The mean apparent volume of distribution was 0.67 l/kg, and the mean body clearance was 0.15 l/kg/h. The extent of binding of FLO to bovine plasma proteins was determined in vitro at concentrations of 5 micrograms/ml and 50 micrograms/ml by equilibrium dialysis and ultrafiltration. The drug was 18% and 19% bound by equilibrium dialysis, and 23% and 19% bound by ultrafiltration, at 5 micrograms/ml and 50 micrograms/ml, respectively. Phagocytosis of 32phosphorus-labelled Staphylococcus aureus by bovine blood neutrophils was compared in vitro between neutrophils incubated in phosphate-buffered saline alone or in combination with 5, 125, or 1000 micrograms/ml chloramphenicol or FLO. There was no significant effect of chloramphenicol at any concentration. Florfenicol significantly inhibited phagocytosis at all concentrations, but the percentage inhibition was small. The clinical significance, if any, of this effect of FLO remains to be demonstrated.

Blood dyscrasias and topically applied chloramphenicol in ophthalmology

The case reports presented so far, concerning a possible causative relationship between the ophthalmological use of chloramphenicol (O-CAP) and blood dyscrasias, are few in number, not always fully documented and therefore not always convincing. The overall use of this drug in a restricted area in the Netherlands was assessed. On a yearly basis 1:29 people used O-CAP. In a four-year period in the same area, 12 patients with aplastic anemia and 190 patients with other cytopenic dyscrasias were found. After exclusion of all cases with other possible causes of the dyscrasia, e.g. cancer and related therapy, other drugs than O-CAP and infectious diseases, 59 cases remained in which a definite cause could not be established. We traced back the use of O-CAP in the relevant period. No dyscrasias were found that unequivocally were caused by O-CAP. The rate of O-CAP use in the dyscrasia-group was approximately equal to that in the population as a whole. Hence we concluded that it is highly improbable that O-CAP played a part in the etiology of blood dyscrasias in the region. Statistical evaluation of material derived from a much larger region may, however, lead to a more differentiated conclusion.

Electrolytic reduction of nitroheterocyclic drugs leads to biologically important damage in DNA

The effect of electrolytic reduction of nitroimidazole drugs on biologically active DNA was studied. The results show that reduction of the drugs in the presence of DNA affects inactivation for both double-stranded (RF) and single-stranded phi X174 DNA. However, stable reduction products did not make a significant contribution to the lethal damage in DNA. This suggests that probably a short-lived intermediate of reduction of nitro-compounds is responsible for damage to DNA.

Malabsorption due to selected oral antibiotics

This article represents an overview of recent research conducted on antibiotic-induced malabsorption in calves. The authors feel strongly that this work identifies a serious and ill-defined problem in the management of neonatal calves. Too often the solution utilized by veterinarian and stockmen for controlling neonatal diarrhea has been to administer oral antibiotics. In many cases, this has been done on the basis of antibiotic sensitivity testing, an approach that seems appropriate. Unfortunately, little consideration has been given to the relative sensitivity of the neonatal intestinal mucosa, with its very rapid turnover, to the potentially detrimental effects of oral antimicrobial therapy. The data that we have collected over the past 3 years conclusively demonstrate that high levels of four commonly used oral antibiotics, especially neomycin and chloramphenicol but also tetracycline and ampicillin, can cause a malabsorption diarrhea in normal calves. This action is not due to viral agents or overgrowth of resistant microbes but is the result of direct modification of the intestinal mucosa. Extrapolation of these data to different dose levels may not be accurate. Oral antibiotics may be of value in treating neonatal enteritis. Conversely, there can be too much of a good thing, and many cases of chronic diarrhea following use of oral antibiotics may be the result of an overenthusiastic and prolonged dosage regimen. Be cautious!