Effect of streptomycin administration on colonization resistance to Salmonella typhimurium in mice

Prolonged impact of antibiotics on intestinal microbial ecology and susceptibility to enteric Salmonella infection

The impact of antibiotics on the host's protective microbiota and the resulting increased susceptibility to mucosal infection are poorly understood. In this study, antibiotic regimens commonly applied to murine enteritis models are used to examine the impact of antibiotics on the intestinal microbiota, the time course of recovery of the biota, and the resulting susceptibility to enteric Salmonella infection. Molecular analysis of the microbiota showed that antibiotic treatment has an impact on the colonization of the murine gut that is site and antibiotic dependent. While combinations of antibiotics were able to eliminate culturable bacteria, none of the antibiotic treatments were effective at sterilizing the intestinal tract. Recovery of total bacterial numbers occurs within 1 week after antibiotic withdrawal, but alterations in specific bacterial groups persist for several weeks. Increased Salmonella translocation associated with antibiotic pretreatment corrects rapidly in association with the recovery of the most dominant bacterial group, which parallels the recovery of total bacterial numbers. However, susceptibility to intestinal colonization and mucosal inflammation persists when mice are infected several weeks after withdrawal of antibiotics, correlating with subtle alterations in the intestinal microbiome involving alterations of specific bacterial groups. These results show that the colonizing microbiotas are integral to mucosal host protection, that specific features of the microbiome impact different aspects of enteric Salmonella pathogenesis, and that antibiotics can have prolonged deleterious effects on intestinal colonization resistance.

A mouse model for characterization of gastrointestinal colonization rates among environmental Aeromonas isolates

The colonization rates of 10 different environmental Aeromonas isolates were determined using a novel mouse-streptomycin pretreatment method. As demonstrated, alterations to the colon flora of mice pretreated with streptomycin allowed transient colonization by bacterial species normally excluded by host competition. A novel procedure is described for determining the colonization abilities of Aeromonas isolates under these conditions. The colonization rates of A. salmonicida, A. encheleia, and A. allosaccharophila were either negative or occurred randomly at low levels with respect to concentrations of the dosage consumed by the animals. In contrast, A. hydrophila, A. veronii biovar sobria, and A. caviae exhibited relatively high rates of mouse colon tissue colonization.

Enteric salmonellosis disrupts the microbial ecology of the murine gastrointestinal tract

The commensal microbiota protects the murine host from enteric pathogens. Nevertheless, specific pathogens are able to colonize the intestinal tract and invade, despite the presence of an intact biota. Possibly, effective pathogens disrupt the indigenous microbiota, either directly through pathogen-commensal interaction, indirectly via the host mucosal immune response to the pathogen, or by a combination of these factors. This study investigates the effect of peroral Salmonella enterica serovar Typhimurium infection on the intestinal microbiota. Since the majority of the intestinal microbiota cannot be cultured by conventional techniques, molecular approaches using 16S rRNA sequences were applied. Several major bacterial groups were assayed using quantitative PCR. Administration of either the 50% lethal dose (LD(50)) or 10x LD(50) of Salmonella enterica serovar Typhimurium caused changes in the microbiota throughout the intestinal tract over the time course of infection. A 95% decrease in total bacterial number was noted in the cecum and large intestine with 10x LD(50) S. enterica serovar Typhimurium challenge at 7 days postinfection, concurrent with gross evidence of diarrhea. In addition, alterations in microbiota composition preceded the onset of diarrhea, suggesting the involvement of pathogen-commensal interactions and/or host responses unrelated to diarrhea. Microbiota alterations were not permanent and reverted to the microbiota of uninfected mice by 1 month postinfection. Infection with a Salmonella pathogenicity island 1 (SPI1) mutant did not result in microbiota alterations, while SPI2 mutant infections triggered partial changes. Neither mutant was capable of prolonged colonization or induction of mucosal inflammation. These data suggest that several Salmonella virulence factors, particularly those involved in the local mucosal host response, are required for disruption of the intestinal ecosystem.

Multidrug-resistant Salmonella enterica serotype Typhimurium associated with pet rodents

BACKGROUND

An estimated 1.4 million salmonella infections occur annually in the United States. The majority of these infections are foodborne, but many are acquired by contact with animals. In August 2004, isolates of Salmonella enterica serotype Typhimurium, which were indistinguishable from one another by pulsed-field gel electrophoresis (PFGE), were obtained from eight hamsters from a Minnesota pet distributor. We conducted an investigation to determine whether human cases of salmonella could be linked to this rodent-borne strain.

METHODS

To identify cases of human infection with S. enterica serotype Typhimurium potentially related to pet rodents, we reviewed salmonella PFGE patterns submitted to the National Molecular Subtyping Network for Foodborne Disease Surveillance. Patients with isolates matching the hamster strain were interviewed about exposure to pet rodents. Implicated rodents were traced to pet stores, distributors, and breeders.

RESULTS

We identified matching S. enterica serotype Typhimurium isolates from 28 patients in whom the onset of illness occurred between December 2003 and September 2004. Of 22 patients (or in the case of children, their parents) interviewed, 13 patients (59%) in 10 states reported exposure to pet hamsters, mice, or rats, and 2 (9%) had secondary infections. The median age of the 15 patients with primary or secondary rodent exposure was 16 years, and 6 patients (40%) were hospitalized. Thirteen associated pet stores supplied by seven distributors were identified in 10 states. No single source of the rodents was identified. The outbreak strain of S. enterica serotype Typhimurium was cultured from a patient's pet mouse and from seven hamsters from pet stores. Closely related S. enterica serotype Typhimurium isolates were cultured from rodent cages and reusable transport containers at a pet distributor. Human, rodent, and environmental isolates were resistant to ampicillin, chloramphenicol, streptomycin, sulfisoxazole, and tetracycline.

CONCLUSIONS

Pet rodents probably are an underrecognized source of human salmonella infection.

Animal models of inflammatory bowel disease: lessons from enteric infections

Mouse models of intestinal inflammation have played a key role in understanding the mechanisms that govern the inflammatory response in the intestine, and in designing new therapeutic strategies in the treatment of patients with inflammatory bowel disease (IBD). Most of these models use chemical challenges, whereas relatively few robust models of intestinal inflammation caused by microbial infection are known. Two common models of infectious murine colitis and typhlitis are infection with the murine epithelial-adherent pathogen, Citrobacter rodentium, and infection of streptomycin-pretreated mice with Salmonella typhimurium. Studies in these models have helped to define the interactions between bacterial pathogens and host immune defenses, thus broadening the understanding of host-microbial interactions in the intestinal tract. Furthermore, such models help to determine the physiologic consequences of neutralizing specific mediators and signaling pathways implicated in inflammation on antimicrobial host defense.

Chronic Salmonella enterica serovar Typhimurium-induced colitis and cholangitis in streptomycin-pretreated Nramp1+/+ mice

Salmonella enterica subspecies 1 serovar Typhimurium is an enteric bacterial pathogen infecting a broad range of hosts. In susceptible Nramp1(-/-) (Slc11alpha1(-/-)) mice, serovar Typhimurium cannot efficiently colonize the intestine but causes a systemic typhoid-like infection. However, after pretreatment with streptomycin, these susceptible (C57BL/6 and BALB/c) mice develop acute serovar Typhimurium-induced colitis (M. Barthel et al., Infect. Immun. 71:2839-2858, 2003). It was not clear whether resistant Nramp1(+/+) (Slc11alpha1(+/+)) mouse strains would similarly develop colitis. Here we compared serovar Typhimurium infection in streptomycin-pretreated susceptible (C57BL/6) and resistant (DBA/2 and 129Sv/Ev) mouse strains: We found that acute colitis (days 1 and 3 postinfection) is strikingly similar in susceptible and resistant mice. In 129Sv/Ev mice we followed the serovar Typhimurium infection for as long as 6 weeks. After the initial phase of acute colitis, these animals developed chronic crypt-destructive colitis, including ulceration, crypt abscesses, pronounced mucosal and submucosal infiltrates, overshooting regeneration of the epithelium, and crypt branching. Moreover, we observed inflammation of the gall duct epithelium (cholangitis) in the 129Sv/Ev mice between days 14 and 43 of infection. Cholangitis was not attributable to side effects of the streptomycin treatment. Furthermore, chronic infection of 129Sv/Ev mice in a typhoid fever model did not lead to cholangitis. We propose that streptomycin-pretreated 129Sv/Ev mice provide a robust murine model for chronic enteric salmonellosis including complications such as cholangitis.

A mouse model for S. typhimurium-induced enterocolitis

Salmonella typhimurium has emerged as a model pathogen that manipulates host cells in a complex fashion, thus causing disease. In humans, S. typhimurium causes acute intestinal inflammation. Intriguingly, type III secreted virulence proteins have a central role in this process. At the cellular level, the functions of these factors are well characterized; at present, animal models are required for elucidating how these factors trigger inflammatory disease in vivo. Calf infection models have been employed successfully and, recently, a mouse model was identified: in streptomycin-pretreated mice, S. typhimurium causes acute colitis. This mouse model provides a new avenue for research into acute intestinal inflammation because it enables the manipulation and dissection of both the bacterial and host contributions to the disease in unsurpassed detail.

Salmonella enterica serovar Typhimurium pathogenicity island 2 is necessary for complete virulence in a mouse model of infectious enterocolitis

Salmonella species cause a wide range of disease in multiple hosts. Salmonella enterica serovar Typhimurium causes self-limited intestinal disease in humans and systemic typhoid-like illness in susceptible mice. The prevailing dogma in murine S. enterica serovar Typhimurium pathogenesis is that distinct virulence mechanisms-Salmonella pathogenicity islands 1 and 2 (SPI1 and SPI2)-perform distinct roles in pathogenesis, the former being important for invasion and intestinal disease and the latter important for intracellular survival and systemic persistence and disease. Although evidence from bovine infections has suggested that SPI2 has a role in ileal disease, there is no evidence that SPI2 is important for inflammation in a disease that more closely recapitulates human colitis. Using S. enterica serovar Typhimurium strains that lack functional type III secretion systems, we demonstrate that SPI2 is essential for complete virulence in murine infectious enterocolitis. Using a recently characterized murine model (M. Barthel,S. Hapfelmeier, L. Quintanilla-Martinez, M. Kremer, M. Rohde, M. Hogardt, K. Pfeffer, H. Russmann, and W. D. Hardt, Infect. Immun. 71:2839-2858, 2003), we demonstrate that SPI1 mutants are unable to cause intestinal disease 48 h after infection and that SPI2-deficient bacteria also cause significantly attenuated typhlitis. We show that at the peak of inflammation in the cecum, SPI2 mutants induce diminished intercellular adhesion molecule 1 expression and neutrophil recruitment but that wild-type and mutant Salmonella are similarly distributed in the lumen of the infected organ. Finally, we demonstrate that attenuation of intestinal inflammation is accompanied by resolution of typhlitis in the mutant, but not wild-type, infections. Collectively, these results indicate that SPI2 is needed for enterocolitis, as well as for systemic disease.

Cryptosporidium and host resistance: historical perspective and some novel approaches

Cryptosporidium parvum is recognized as a major cause of diarrheal disease in neonatal bovine calves. In addition, this protozoan parasite has emerged as an important cause of disease in both immunocompromised and immunocompetent humans. Despite years of research, no consistently effective means of prevention or treatment are readily available for cryptosporidiosis in any species. Infection through ingestion of contaminated water has been widely documented; C. parvum was reported to be responsible for the largest waterborne outbreak of infectious disease in US history. In addition to its role as a primary disease agent, C. parvum has potential to initiate or exacerbate other gastrointestinal disorders, such as inflammatory bowel disease. Thus, control of C. parvum infection in both animals and humans remains an important objective. Research in our laboratory has focused on understanding mechanisms of resistance to C. parvum. We have demonstrated that acquisition of intestinal flora increases resistance to C. parvum. Substances present in the intestinal mucosa of adult animals can transfer resistance when fed to susceptible infants. Both expression of intestinal enzymes and rate of proliferation of epithelial cells may be altered following C. parvum infection. These and other changes may have profound effects on host resistance to C. parvum.

Pretreatment of mice with streptomycin provides a Salmonella enterica serovar Typhimurium colitis model that allows analysis of both pathogen and host

Salmonella enterica subspecies 1 serovar Typhimurium is a principal cause of human enterocolitis. For unknown reasons, in mice serovar Typhimurium does not provoke intestinal inflammation but rather targets the gut-associated lymphatic tissues and causes a systemic typhoid-like infection. The lack of a suitable murine model has limited the analysis of the pathogenetic mechanisms of intestinal salmonellosis. We describe here how streptomycin-pretreated mice provide a mouse model for serovar Typhimurium colitis. Serovar Typhimurium colitis in streptomycin-pretreated mice resembles many aspects of the human infection, including epithelial ulceration, edema, induction of intercellular adhesion molecule 1, and massive infiltration of PMN/CD18(+) cells. This pathology is strongly dependent on protein translocation via the serovar Typhimurium SPI1 type III secretion system. Using a lymphotoxin beta-receptor knockout mouse strain that lacks all lymph nodes and organized gut-associated lymphatic tissues, we demonstrate that Peyer's patches and mesenteric lymph nodes are dispensable for the initiation of murine serovar Typhimurium colitis. Our results demonstrate that streptomycin-pretreated mice offer a unique infection model that allows for the first time to use mutants of both the pathogen and the host to study the molecular mechanisms of enteric salmonellosis.

Lack of flagella disadvantages Salmonella enterica serovar Enteritidis during the early stages of infection in the rat

The roles of flagella and five fimbriae (SEF14, SEF17, SEF21, pef, lpf) in the early stages (up to 3 days) of Salmonella enterica serovar Enteritidis (S. Enteritidis) infection have been investigated in the rat. Wild-type strains LA5 and S1400 (fim+/fla+) and insertionally inactivated mutants unable to express the five fimbriae (fim-/fla+), flagella (fim+/fla-) or fimbriae and flagella (fim-/fla-) were used. All wild-type and mutant strains were able to colonize the gut and spread to the mesenteric lymph nodes, liver and spleen. There appeared to be little or no difference between the fim-/fla+ and wild-type (fim+/fla+) strains. In contrast, the numbers of aflagellate (fim+/fla- or fim-/fla-) salmonella in the liver and spleen were transiently reduced. In addition, fim+/fla- or fim-/fla- strains were less able to persist in the upper gastrointestinal tract and the inflammatory responses they elicited in the gut were less severe. Thus, expression of SEF14, SEF17, SEF21, pef and lpf did not appear to be a prerequisite for induction of S. Enteritidis infection in the rat. Deletion of flagella did, however, disadvantage the bacterium. This may be due to the inability to produce or release the potent immunomodulating protein flagellin.

Transfer of the pheromone-inducible plasmid pCF10 among Enterococcus faecalis microorganisms colonizing the intestine of mini-pigs

A new animal model, the streptomycin-treated mini-pig, was developed in order to allow colonization of defined strains of Enterococcus faecalis in numbers sufficient to study plasmid transfer. Transfer of the pheromone-inducible pCF10 plasmid between streptomycin-resistant strains of E. faecalis OG1 was investigated in the model. The plasmid encodes resistance to tetracycline. Numbers of recipient, donor, and transconjugant bacteria were monitored by selective plating of fecal samples, and transconjugants were subsequently verified by PCR. After being ingested by the mini-pigs, the recipient strain persisted in the intestine at levels between 10(6) and 10(7) CFU per g of feces throughout the experiment. The donor strain, which carried different resistance markers but was otherwise chromosomally isogenic to the recipient strain, was given to the pigs 3 weeks after the recipient strain. The donor cells were initially present in high numbers (10(6) CFU per g) in feces, but they did not persist in the intestine at detectable levels. Immediately after introduction of the donor bacteria, transconjugant cells appeared and persisted in fecal samples at levels between 10(3) and 10(4) CFU per g until the end of the experiment. These observations showed that even in the absence of selective tetracycline pressure, plasmid pCF10 was transferred from ingested E. faecalis cells to other E. faecalis organisms already present in the intestinal environment and that the plasmid subsequently persisted in the intestine.

Invasion genes are not required for Salmonella enterica serovar typhimurium to breach the intestinal epithelium: evidence that salmonella pathogenicity island 1 has alternative functions during infection

Salmonella enterica serovar Typhimurium invasion genes are necessary for bacterial invasion of intestinal epithelial cells and are thought to allow salmonellae to enter and cross the intestinal epithelium during infection. Many invasion genes are encoded on Salmonella pathogenicity island 1 (SPI1), and their expression is activated by HilA, a transcription factor also encoded on SPI1. We have studied the role of Salmonella invasion genes during infection of mice following intragastric inoculation. We have found that strains containing a mutation in hilA or invG were recovered from the intestinal contents, intestinal tissues, and systemic tissues at a lower frequency than their parental wild-type strain. In contrast, a strain in which SPI1 is deleted was recovered from infected mice at a frequency similar to that of its parental wild-type strain. The DeltaSPI1 phenotype indicates that S. enterica does not require invasion genes to cross the intestinal epithelium and infect systemic tissues. This result has forced us to reconsider the long-held belief that invasion genes directly mediate bacterial infection of the intestinal mucosa and traversion of the intestinal barrier during infection. Instead, our results suggest that hilA is required for bacterial colonization of the host intestine. The seemingly contradictory phenotype of the DeltaSPI1 mutant suggests that deletion of another gene(s) encoded on SPI1 suppresses the hilA mutant defect. We propose a model for S. enterica pathogenesis in which hilA and invasion genes are required for salmonellae to overcome a host clearance response elicited by another SPI1 gene product(s).

Increased antibody production against gut-colonizing Escherichia coli in the presence of the anaerobic bacterium Peptostreptococcus

Germ-free rats were colonized with E. coli alone, or with E. coli plus Lactobacillus acidophilus and a strain of the obligate anaerobic gram-positive species, Peptostreptococcus. The presence of Peptostreptococcus reduced translocation of E. coli, but increased the serum antibody response to E. coli antigen. Whereas the immunoglobulin G (IgG) anti-E. coli antibodies largely represented cross-reactive antibodies, those of the immunoglobulin M (IgM) isotype represented true anti-E. coli antibodies because they could not be absorbed by L. acidophilus or Peptostreptococcus but could with E. coli. We suggest that peptostreptococci prime the gut immune system to other bacterial antigens and that this could be a mechanism behind the reduced translocation of facultative anaerobes in the presence of obligate anaerobes.

The lpf fimbrial operon mediates adhesion of Salmonella typhimurium to murine Peyer's patches

We investigated the role of the Salmonella typhimurium fimbrial operon formed by the genes lpfABCDE in infection of mice. A mutant in lpfC, the gene encoding the fimbrial outer membrane usher, had an approximately 5-fold increased 50% lethal dose when administered orally to mice. When mice were infected with a mixture of the lpfC mutant and isogenic wild-type S. typhimurium, the lpfC mutant was recovered in lower numbers from Peyer's patches, mesenteric lymph nodes, liver, and spleen. In an organ culture model using murine intestinal loops, lpfC mutants were shown to be associated in lower numbers than wild-type bacteria with Peyer's patches but not with villous intestine. The defect of the lpfC mutant in adhesion to Peyer's patches could be complemented by introducing lpfABCDE on a cosmid. Similarly, heterologous expression of the Salmonella lpf operon in Escherichia coli resulted in an increased adhesion to histological thin sections of Peyer's patch lymph follicles. Electron microscopic analysis of histological sections taken from Peyer's patches after intragastric infection of mice showed that, in contrast to the S. typhimurium wild type, the isogenic lpfC mutant did not destroy M cells of the follicle-associated epithelium. These data show that the Salmonella lpf operon is involved in adhesion to murine Peyer's patches.

Effects of subinhibitory concentrations of antibiotics on biological properties of Salmonella typhimurium

We followed the effects of subinhibitory concentrations (sub-MICs) of 7 antibiotics (ticarcilin, cefotaxim, streptomycin, gentamicin, ciprofloxacin, pefloxacin, mitomycin C) on the sensitivity of a Salmonella typhimurium strain to standard bacteriophages, on the phage DNA as well as on the factors of virulence (permeability and cytotoxic activity). The phage type was not changed by the sub-MICs of the tested antibiotics. However, differences were found in culture filtrates prepared from the bacterial suspensions of the strain cultivated with the sub-MICs. Marked inducing effects on phage DNA were exhibited by mitomycin C (1/2, 1/4, 1/8 of the MIC), pefloxacin (1/2, 1/4, 1/8 of the MIC) and ciprofloxacin (1/2, 1/4, weakly also 1/8 of the MIC). Ticarcilin (1/2 of the MIC), like the aminoglycosides streptomycin and gentamicin (1/2, 1/4, 1/8 of the MIC), had a weak effect. Sub-MICs of the studied antibiotics (with the exception of 1/8 of the MIC of ciprofloxacin and 1/4 of the MIC of ticarcilin) decreased the permeability reaction in rabbit skin. Most effective was streptomycin (1/2 of the MIC). Sub-MICs of the tested antibiotics (with the exception of 1/4 and 1/8 of the MIC of ciprofloxacin and 1/4 of the MIC of pefloxacin) caused also an inhibition of the factor responsible for morphological changes on Vero cells. Gentamicin and streptomycin were effective at all the sub-MICs tested.

Intestinal, segmented, filamentous bacteria

Segmented, filamentous bacteria (SFBs) are autochthonous, apathogenic bacteria, occurring in the ileum of mice and rats. Although the application of formal taxonomic criteria is impossible due to the lack of an in vitro technique to culture SFBs, microbes with a similar morphology, found in the intestine of a wide range of vertebrate and invertebrate host species, are considered to be related. SFBs are firmly attached to the epithelial cells of the distal ileal mucosa, their preferential ecological niche being the epithelium covering the Peyer's patches. Electron microscopic studies have demonstrated a considerable morphological diversity of SFBs, which may relate to different stages of a life cycle. Determinants of SFB colonization in vivo are host species, genotypical and phenotypical characteristics of the host, diet composition, environmental stress and antimicrobial drugs. SFBs can survive in vitro incubation, but do not multiply. On the basis of their apathogenic character and intimate relationship with the host, it is suggested that SFBs contribute to development and/or maintenance of host resistance to enteropathogens.

The Salmonella typhimurium locus mviA regulates virulence in Itys but not Ityr mice: functional mviA results in avirulence; mutant (nonfunctional) mviA results in virulence

The virulent Salmonella typhimurium strain WB600 carries the mviA allele of the gene mouse virulence A. As shown here, the virulent phenotype of WB600 is the result of a nonfunctional mviA gene. As compared to the functional allele mviA+, mviA increases virulence in Itys mice, but not in Ityr mice. A specific BglII site, mviA4185, between osmZ and galU, located at approximately 35 min on the salmonella chromosome, was within mviA. Insertion of an antibiotic cassette in the mviA4185 site of mviA+ or the homologous mviA4093 site of mviA DNA resulted in virulence when either cassette was recombined into the chromosome. When mviA and mviA+ were both expressed in the same strain with one carried in the chromosome and the other on a plasmid, avirulence was dominant. Replacement of the mviA allele of strain WB600 using P22 transductions of linked antibiotic cassettes cloned into the chromosome of virulent S. typhimurium strains (SR-11, TML, SL1344, C5, ATCC14028, W118-2, and WB600) showed that all but WB600 contained the avirulent mviA+ allele. Southern hybridizations provided no evidence for a second mviA allele anywhere in the genome of the six non-WB600 strains.

Influence of antibiotics on intestinal tract survival and translocation of environmental Pseudomonas species

The environmental release of microorganisms has prompted the investigation of potential health effects associated with their release. In this study, survival and translocation to the spleen and liver of several environmental Pseudomonas spp. were investigated in antibiotic-treated mice. Pseudomonas aeruginosa BC16 and P. maltophilia BC6, isolated from a commercial product for polychlorinated biphenyl degradation; P. aeruginosa AC869, a 3,5-dichlorobenzoate degrader; and P. cepacia AC1100, an organism that metabolizes 2,4,5-trichlorophenoxyacetic acid were examined for their survival capabilities in the intestines of mice dosed with clindamycin, kanamycin, rifampin, or spectinomycin. A mouse intestinal isolate, strain PAMG, was included in the study. Following antibiotic pretreatment (1 mg twice daily for 3 days), mice were dosed by gavage with 10(9) CFU of each Pseudomonas strain. At the end of the 5-day test period, strains AC869 and PAMG survived in kanamycin-, rifampin-, spectinomycin-, and clindamycin-treated animals. A statistically significant (P less than 0.05) increase in survival of strain PAMG was observed in clindamycin-, kanamycin-, and spectinomycin-treated mice for the test period. Treatment with clindamycin or rifampin increased (P less than 0.05) survival of strain BC6, an organism resistant to both antibiotics. However, strain BC6 was detected only in rifampin-treated mice at the end of the 5-day test period. Strain BC16, a clindamycin-resistant strain, was detected in clindamycin-treated mice and the untreated control animals. Rifampin had a negative effect (P less than 0.05) on strain AC869 and PAMG survival. Translocation to the spleen was observed in spectinomycin- and clindamycin-treated mice but was not detected in kanamycin- or rifampin-treated animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Colonization of mice by Campylobacter jejuni

Both streptomycin-treated and untreated Swiss white mice were irregularly colonized when challenged orogastrically with between 1 and 10(11) viable organisms of either of two strains of Campylobacter jejuni. The organisms were occasionally recovered from portions of the intestinal tracts of these animals in numbers ranging from 10(1) to 10(3)/g when the challenge doses were 10(10) or more. When germfree mice were challenged with 10(8) organisms of either strain, the entire intestinal tracts of all the animals were colonized with C. jejuni in numbers ranging from 10(4) to 10(9)/g. The ceca were most heavily colonized. Both strains of C. jejuni multiplied anaerobically in brucella broth, except when the broth contained 60.80 mu eq of total volatile fatty acids (VFA) per ml at pH 6.75, simulating conditions in the ceca of untreated mice, or when it contained 21.63 mu eq/ml at pH 7.04, simulating conditions in the ceca of streptomycin-treated mice. Active multiplication occurred, however, in brucella broth without VFA at pH 7.02 that was incubated microaerobically, simulating conditions in the ceca of germfree mice. The results suggest that VFA operating under anaerobic conditions present in the intestinal tract of both streptomycin-treated and untreated conventional mice interfere with the multiplication of C. jejuni. The organisms actively multiply, on the other hand, in the absence of VFA at the higher oxidation-reduction potential of the intestinal tract of germfree mice.

Susceptibility of germfree or antibiotic-treated adult mice to Cryptosporidium parvum

Adult mice are more resistant than neonatal mice to intestinal colonization with the protozoan parasite Cryptosporidium parvum. Development of a mature intestinal flora may play a role in this resistance. We compared susceptibilities to colonization with C. parvum in adult conventional mice, adult germfree mice, and adult conventional mice treated with oral antibiotics to deplete the intestinal flora. Germfree mice of both CD1 and BALB/c strains were colonized at day 7 following inoculation with C. parvum oocysts isolated from the feces of an infected, diarrheic calf. Age-matched conventional mice of the same strains were comparatively resistant to colonization. Conventional mice treated with antibiotics remained resistant to colonization. These results suggest that the microflora in the intestine was not the sole determinant of resistance or susceptibility to colonization. The germfree adult mouse as an experimental model of cryptosporidiosis is discussed.

Impact of LY146032 on Streptococcus (Enterococcus) faecalis translocation in mice

The susceptibility of Swiss White mice to colonization with Streptococcus (Enterococcus) faecalis was greatly increased when the animals were given 5 mg of streptomycin sulfate per ml in their drinking water. One week after initiation of streptomycin treatment, the mice were challenged orogastrically with graded doses of streptomycin-resistant S. faecalis. The number of S. faecalis cells required to implant the intestinal tract of 50% of untreated mice was 2.9 X 10(9), but was only 4.8 X 10(3) for streptomycin-treated animals. When both groups of mice were challenged orogastrically with 4.6 X 10(6) viable S. faecalis cells, the cecum and small intestine of 100% of the streptomycin-treated animals, but only 10% of the untreated animals, were colonized with the organism. Similarly, translocation of S. faecalis to extraintestinal sites occurred in a majority of streptomycin-treated mice, but in only a small number of untreated mice. Subcutaneous administration of the experimental antibiotic LY146032 (Eli Lilly & Co., Indianapolis, Ind.) to streptomycin-treated mice concomitant with orogastric challenge with 5.5 X 10(5) viable S. faecalis cells resulted in a significant decrease in the incidence of intestinal colonization by the organism, a significant reduction in S. faecalis populations, and the absence of the organism in the liver, spleen, and heart. However, once intestinal colonization had occurred and extraintestinal infections were established, LY146032 did not significantly reduce S. faecalis populations or ameliorate the infections. We conclude that LY146032 effectively prevents translocation of S. faecalis from the intestinal tract of mice but does not resolve established extraintestinal infections.

Evidence for the phagocytic transport of intestinal particles in dogs and rats

Fluorescent latex beads of two different colors were implanted into separate intestinal segments in individual dogs and rats. Mesenteric lymph node phagocytes subsequently contained multiple beads of one or the other color but rarely both colors, indicating that intestinal phagocytes transported the latex beads to the draining lymph node. Fluorescent labeled Escherichia coli was implanted into rat ligated intestinal segments, and rare mesenteric lymph node phagocytes subsequently contained fluorescent bacteria, suggesting that intestinal bacteria might be transported in the same manner as inert latex beads.

Role of anaerobic flora in the translocation of aerobic and facultatively anaerobic intestinal bacteria

It is thought that the normal enteric microflora acts not only to prevent intestinal colonization but also to prevent subsequent systemic dissemination of ingested, potentially pathogenic bacteria. To determine the relative roles of specific components of the intestinal bacterial flora in bacterial translocation out of the gut, mice were given various antimicrobial agents to selectively eliminate specific groups of intestinal bacteria. The cecal flora and the translocating bacteria in mesenteric lymph nodes were monitored both before and after oral inoculation with antibiotic-resistant Escherichia coli C25. Orally administered streptomycin selectively eliminated cecal facultative gram-negative bacilli, orally administered bacitracin-streptomycin eliminated all cecal bacterial species except low numbers of aerobic sporeformers, and parenterally administered metronidazole selectively eliminated cecal anaerobic bacteria. Compared with control mice, only metronidazole-treated mice had significantly increased rates of dissemination of intestinal bacteria into mesenteric lymph nodes, indicating that the exclusive absence of anaerobic bacteria facilitated the translocation of the intestinal facultative bacteria. In a parallel experiment with streptomycin-resistant E. coli C25 as a marker, parallel results were obtained. Metronidazole increased the translocation of the marker strain and the indigenous strains of intestinal bacteria. Thus, anaerobes appeared to play a key role in confining indigenous bacteria to the gut. However, intestinal colonization and translocation of E. coli C25 occurred most readily after bacitracin-streptomycin treatment, suggesting that in addition to anaerobic bacteria, other bacterial groups may play a role in limiting the intestinal colonization and extraintestinal dissemination of E. coli C25.

Factors responsible for increased susceptibility of mice to intestinal colonization after treatment with streptomycin

Streptomycin sulfate (5 mg/ml) was added to the drinking water of Swiss white mice. After treatment for 1 week, the mice were challenged orogastrically with 10(8) Pseudomonas aeruginosa cells. The organism failed to multiply in the intestinal tract of either treated or untreated animals, but could be recovered from contents and tissues after 48 h. In a previous study, Salmonella typhimurium was shown to multiply in the intestines of streptomycin-treated but not untreated mice when 10(3) organisms were used as inoculum. Streptomycin administration had little effect on Eh, protein or carbohydrate concentrations of cecal contents, or intestinal motility. However, it caused a statistically significant increase in water content and pH of contents and a decrease in the concentrations of acetic, propionic, butyric, and valeric acids. S. typhimurium multiplied in pooled cecal contents obtained from both streptomycin-treated and untreated animals, but its multiplication rate and total populations were significantly greater in contents from treated animals. P. aeruginosa did not multiply in contents from either treated or untreated mice. Similar results were obtained when the organisms were inoculated into nutrient broth adjusted to simulate the pH levels and volatile fatty acid (VFA) concentrations in cecal contents of treated and untreated mice. The addition of brain heart infusion broth to cecal contents from untreated animals, in concentrations that support multiplication of S. typhimurium and P. aeruginosa, did not reverse inhibition. The addition of VFA to cecal contents from treated animals to equal the concentration in cecal contents from untreated animals caused inhibition of a magnitude observed in cecal contents from untreated animals. The results indicate that VFA operating at the pH level of cecal contents of conventional mice inhibit the multiplication of both S. typhimurium and P. aeruginosa and restrict colonization of the intestine by these organisms. The decrease in VFA concentrations that occurs as a result of streptomycin administration adequately explains the increased susceptibility of treated mice to colonization with S. typhimurium. It does not explain the increased susceptibility of treated mice to P. aeruginosa colonization, however.