Archaean metabolic evolution of microbial mats

Microbial mats of coexisting bacteria and archaea date back to the early Archaean: many of the major steps in early evolution probably took place within them. The earliest mats may have formed as biofilms of cooperative chemolithotrophs in hyperthermophile settings, with microbial exploitation of diversifying niches. Anoxygenic photosynthesis using bacteriochlorophyll could have allowed mats, including green gliding bacteria, to colonize anaerobic shallow-water mesothermophile habitats. Exploitation of the Calvin–Benson cycle by purple bacteria allowed diversification of microbial mats, with some organisms in more aerobic habitats, while green sulphur bacteria specialized in anaerobic niches. Cyanobacterial evolution led to more complex mats and plankton, allowing widespread colonization of the globe and the creation of further aerobic habitat. Microbial mat structure may reflect this evolutionary development in broad terms, with anaerobic lower levels occupied by archaeal and bacterial respirers, fermenters and green bacteria, while the higher levels contain aerobic purple bacteria and are dominated by cyanobacteria. A possible origin of eukaryotes is from a fusion of symbiotic partners living across a redox boundary in a mat. The geological record of Archaean mats may be present as isotopic fingerprints: with the presence of cyanobacteria, mats may have had a nearly modern structure as early as 3.5 Ga ago (1 Ga = 10⁹ years).     

In situ analysis of oxygen consumption and diffusive transport in high‐temperature acidic iron‐oxide microbial mats

The role of dissolved oxygen as a principal electron acceptor for microbial metabolism was investigated within Fe(III)‐oxide microbial mats that form in acidic geothermal springs of Yellowstone National Park (USA). Specific goals of the study were to measure and model dissolved oxygen profiles within high‐temperature (65–75°C) acidic (pH = 2.7–3.8) Fe(III)‐oxide microbial mats, and correlate the abundance of aerobic, iron‐oxidizing Metallosphaera yellowstonensis organisms and mRNA gene expression levels to Fe(II)‐oxidizing habitats shown to consume oxygen. In situ oxygen microprofiles were obtained perpendicular to the direction of convective flow across the aqueous phase/Fe(III)‐oxide microbial mat interface using oxygen microsensors. Dissolved oxygen concentrations dropped from ～ 50–60 μM in the bulk‐fluid/mat surface to below detection (< 0.3 μM) at a depth of ～ 700 μm (～ 10% of the total mat depth). Net areal oxygen fluxes into the microbial mats were estimated to range from 1.4–1.6 × 10^?4 μmol cm^?2 s^?1. Dimensionless parameters were used to model dissolved oxygen profiles and establish that mass transfer rates limit the oxygen consumption. A zone of higher dissolved oxygen at the mat surface promotes Fe(III)‐oxide biomineralization, which was supported using molecular analysis of Metallosphaera yellowstonensis 16S rRNA gene copy numbers and mRNA expression of haem Cu oxidases (FoxA) associated with Fe(II)‐oxidation.     

Proliferation of Purple Sulphur Bacteria at the Sediment Surface Affects Intertidal Mat Diversity and Functionality

There is a relative absence of studies dealing with mats of purple sulphur bacteria in the intertidal zone. These bacteria display an array of metabolic pathways that allow them to disperse and develop under a wide variety of conditions, making these mats important in terms of ecosystem processes and functions. Mass blooms of purple sulphur bacteria develop during summer on sediments in the intertidal zone especially on macroalgal deposits. The microbial composition of different types of mats differentially affected by the development of purple sulphur bacteria was examined, at low tide, using a set of biochemical markers (fatty acids, pigments) and composition was assessed against their influence on ecosystem functions (sediment cohesiveness, CO₂ fixation). We demonstrated that proliferation of purple sulphur bacteria has a major impact on intertidal mats diversity and functions. Indeed, assemblages dominated by purple sulphur bacteria (Chromatiaceae) were efficient exopolymer producers and their biostabilisation potential was significant. In addition, the massive growth of purple sulphur bacteria resulted in a net CO₂ degassing whereas diatom dominated biofilms represented a net CO₂ sink.     

Estimation of dominant microbial population sizes in the anaerobic granular sludge of a full-scale UASB treating streptomycin wastewater by PCR-DGGE

Although the dominant members of microbial communities in wastewater bio-treatment systems were often paid attention due to their possible important roles in treatment performance, their population sizes, especially the unculturable species, were still little known. Then PCR-DGGE was used in an attempt to estimate the dominant microbial population sizes in the anaerobic granular sludge treating streptomycin wastewater, coupled with an inoculated strain (Esherichia coli) with known population sizes as an internal standard. The results indicated that the band intensities of the inoculated strain in DGGE profiles showed good correlation with population sizes. Then it was possible to estimate the dominant microbial population sizes by means of comparing their DGGE band intensities with the inoculated strain. The estimated results demonstrated that the sizes of major dominant microbial populations in the sludge sample were at the level of 10⁷–10⁸ CFU/g. The sizes of secondary dominant microbial populations were at the level of 10⁵–10⁶ CFU/g. The microbial populations with the size level lower than 10³ CFU/g were undetectable by PCR-DGGE. These results provided a potential approach to evaluate dominant microbial population sizes in complex microbial communities.     

Cultivable microscopic fungi from an underground chemosynthesis-based ecosystem: a preliminary study

Movile Cave, a unique groundwater ecosystem in southern Romania, was discovered in 1986. This chemoautotrophic cave contains an abundant and diverse fauna with terrestrial and aquatic invertebrate communities, including 33 endemic species. Since its discovery, studies have focused mainly on cave chemoautotrophic bacteria, while the microfungal community has been largely neglected. In this study, we determined the microfungal species living on various substrates in Movile Cave and compared this spectrum with the mycobiota detected outside the cave (outside air-borne and soil-borne microfungi). To investigate all of the niches, we collected samples for two consecutive years from the dry part of the cave (cave air and sediment, corroded limestone walls, isopod feces, and isopod and spider cadavers) and from the post-siphon part of the cave, i.e., Airbell II (sediment and floating microbial mat). A total of 123 microfungal species were identified from among several hundred isolates. Of these, 96 species were only detected in the cave environment and not outside of the cave, while 90 species were from the dry part of the cave and 28 were from Airbell II. The most diverse genera were Penicillium (at least 18 species) and Aspergillus (14 species), followed by Cladosporium (9 species). Surprisingly, high CFU counts of air-borne microfungi were found inside the cave; they were even higher than outside the cave during the first year of investigation.     

Intact Phospholipid and Quinone Biomarkers to Assess Microbial Diversity and Redox State in Microbial Mats

Microbial mats are stratified microbial communities composed by highly inter-related populations and therefore are frequently chosen as model systems to study diversity and ecophysiological strategies. The present study describes an integrated approach to analyze microbial quinones and intact polar lipids (IPLs) in microbial mats within layers as thin as 500 μm by liquid chromatography–tandem mass spectrometry. Quinone profiles revealed important depth-related differences in community composition in two mat systems. The higher abundance of ubiquinones, compared to menaquinones, reflected the clear predominance of microorganisms belonging to aerobic α-, β-, and γ-Proteobacteria in Ebro delta estuarine mats. Hypersaline photosynthetic Camargue mats (France) showed a predominance of menaquinone-9 at the top of the mat, which is consistent with an important contribution of facultative aerobic or anaerobic bacteria in its photic zone. Quinone indices also indicated a higher diversity of non-phototrophs and a more anaerobic character in the hypersaline mats. Besides, the dissimilarity index suggested that the samples were greatly influenced by a depth-related redox state gradient. In the analysis of IPLs, there was a predominance of phosphatidylglycerols and sulfoquinovosyldiacylglycerols, the latter being an abundant biomarker of Cyanobacteria. This combined approach based on quinone and IPL analysis has proven to be a useful method to establish differences in the microbial diversity and redox state of highly structure microbial mat systems at a fine-scale level.     

Comparison between Cultured Small-Intestinal and Fecal Microbiotas in Beagle Dogs

The microbiota of the small intestine is poorly known because of difficulties in sampling. In this study, we examined whether the organisms cultured from the jejunum and feces resemble each other. Small-intestinal fluid samples were collected from 22 beagle dogs with a permanent jejunal fistula in parallel with fecal samples. In addition, corresponding samples from seven of the dogs were collected during a 4-week period (days 4, 10, 14, and 28) to examine the stability of the microbiota. In the jejunal samples, aerobic/facultative and anaerobic bacteria were equally represented, whereas anaerobes dominated in the fecal samples. Despite lower numbers of bacteria in the jejunum (range, 10² to 10⁶ CFU/g) than in feces (range, 10⁸ to 10¹¹ CFU/g), some microbial groups were more prevalent in the small intestine: staphylococci, 64% versus 36%; nonfermentative gram-negative rods, 27% versus 9%; and yeasts, 27% versus 5%, respectively. In contrast, part of the fecal dominant microbiota (bile-resistant Bacteroides spp., Clostridium hiranonis-like organisms, and lactobacilli) was practically absent in the jejunum. Many species were seldom isolated simultaneously from both sample types, regardless of their overall prevalence. In conclusion, the small intestine contains a few bacterial species at a time with vastly fluctuating counts, opposite to the results obtained for the colon, where the major bacterial groups remain relatively constant over time. Qualitative and quantitative differences between the corresponding jejunal and fecal samples indicate the inability of fecal samples to represent the microbiotas present in the upper gut.     

Dynamics of Dimethyl Sulfide in a Marine Microbial Mat

Abstract The microbial mat was chosen as a model ecosystem to study dynamics of dimethyl sulfide (DMS) in marine sediments in order to gain insight into key processes and factors which determine emission rates. A practical advantage, compared to open ocean ecosystems, is that microbial mats contain high biomasses of different functional groups of bacteria involved in DMS dynamics, and that DMS concentrations are generally high enough to allow direct measurement of emission rates. Field data showed that, during the seasonal development of microbial mats, concentrations of chlorophyll a corresponded to dimethylsulfoniopropionate (DMSP). DMSP is an important precursor of DMS. It was demonstrated, with laboratory cultures, that various species of benthic diatoms produce substantial amounts of DMSP. The abundances of aerobic and anaerobic DMS- or DMSO-utilizing bacteria were estimated using the most-probable-number technique. Laboratory experiments with relatively undisturbed sediment cores showed that microbial mats act as a sink for DMS under oxic/light (day) conditions, and as a source of DMS under anoxic/dark (night) conditions. Axenic culture studies with Chromatium vinosum M2 and Thiocapsa pfennigii M8 (isolated from a microbial mat) showed that, under anoxic/light conditions, DMS was quantitatively converted to dimethylsulfoxide (DMSO). T. roseopersicina M11 converted DMSP to DMS and acrylate, apparently without use of either substrate. Received: 5 May 1997; Accepted: 21 August 1997     

Biogeochemistry and community composition of iron- and sulfur-precipitating microbial mats at the Chefren mud volcano (Nile Deep Sea Fan, Eastern Mediterranean)

In this study we determined the composition and biogeochemistry of novel, brightly colored, white and orange microbial mats at the surface of a brine seep at the outer rim of the Chefren mud volcano. These mats were interspersed with one another, but their underlying sediment biogeochemistries differed considerably. Microscopy revealed that the white mats were granules composed of elemental S filaments, similar to those produced by the sulfide-oxidizing epsilonproteobacterium "Candidatus Arcobacter sulfidicus." Fluorescence in situ hybridization indicated that microorganisms targeted by a "Ca. Arcobacter sulfidicus"-specific oligonucleotide probe constituted up to 24% of the total the cells within these mats. Several 16S rRNA gene sequences from organisms closely related to "Ca. Arcobacter sulfidicus" were identified. In contrast, the orange mat consisted mostly of bright orange flakes composed of empty Fe(III) (hydr)oxide-coated microbial sheaths, similar to those produced by the neutrophilic Fe(II)-oxidizing betaproteobacterium Leptothrix ochracea. None of the 16S rRNA gene sequences obtained from these samples were closely related to sequences of known neutrophilic aerobic Fe(II)-oxidizing bacteria. The sediments below both types of mats showed relatively high sulfate reduction rates (300 nmol x cm(-3) x day(-1)) partially fueled by the anaerobic oxidation of methane (10 to 20 nmol x cm(-3) x day(-1)). Free sulfide produced below the white mat was depleted by sulfide oxidation within the mat itself. Below the orange mat free Fe(II) reached the surface layer and was depleted in part by microbial Fe(II) oxidation. Both mats and the sediments underneath them hosted very diverse microbial communities and contained mineral precipitates, most likely due to differences in fluid flow patterns.     

The spring energy budget of the algal mat community in a Crimean hypersaline lake determined by microcalorimetry

The energy contents (standing stock) of the floating mat formed by the green alga Cladophora sivaschensis and the energy transfers through it were quantified for a shallow hypersaline lake (at Cape Khersones, Crimea, Ukraine) during the spring months. Appropriate direct calorimetric techniques were applied to: (i) measure the heat energy dissipated by the mat community and by the free bacterioplankton in the water column below it; and (ii) differentiate between the heat flows by the heterotrophic and the phototrophic components of the community. It was shown that Cladophora biomass reached a peak of 579.5 g C m^–2, contributing more than 99.6% of the total mat community. Throughout the spring, the total bacterial energy transfer (6 to 23 mW m^–2) was as little as 1.1 to 2.6% of the total heat dissipated by the microplankton community. The rest of the estimated heat energy (584 to 1488 mW m^–2) was associated with Cladophora metabolism. In the spring community: (i) the rate of biomass accumulation in the lake photic layer significantly exceeded its heterotrophic mineralisation; (ii) the efficiency of the microbial loop was too low to process even a minor part of the accumulated organic matter. The microcalorimetric technique was shown to be a highly promising approach for further studies of natural microbial mats and biofilms, biological systems with complex metabolism that involves not only aerobic processes but also anaerobic catabolism under local hypoxic/microxic conditions.     

Calcium dynamics in microbialite‐forming exopolymer‐rich mats on the atoll of Kiritimati,Republic of Kiribati,Central Pacific

Microbialite‐forming microbial mats in a hypersaline lake on the atoll of Kiritimati were investigated with respect to microgradients, bulk water chemistry, and microbial community composition. O₂, H₂S, and pH microgradients show patterns as commonly observed for phototrophic mats with cyanobacteria‐dominated primary production in upper layers, an intermediate purple layer with sulfide oxidation, and anaerobic bottom layers with sulfate reduction. Ca²⁺ profiles, however, measured in daylight showed an increase of Ca²⁺ with depth in the oxic zone, followed by a sharp decline and low concentrations in anaerobic mat layers. In contrast, dark measurements show a constant Ca²⁺ concentration throughout the entire measured depth. This is explained by an oxygen‐dependent heterotrophic decomposition of Ca²⁺‐binding exopolymers. Strikingly, the daylight maximum in Ca²⁺ and subsequent drop coincides with a major zone of aragonite and gypsum precipitation at the transition from the cyanobacterial layer to the purple sulfur bacterial layer. Therefore, we suggest that Ca²⁺ binding exopolymers function as Ca²⁺ shuttle by their passive downward transport through compression, triggering aragonite precipitation in the mats upon their aerobic microbial decomposition and secondary Ca²⁺ release. This precipitation is mediated by phototrophic sulfide oxidizers whose action additionally leads to the precipitation of part of the available Ca²⁺ as gypsum.     

Anaerobic/aerobic conditions and biostimulation for enhanced chlorophenols degradation in biocathode microbial fuel cells

Anaerobic/aerobic conditions affected bacterial community composition and the subsequent chlorophenols (CPs) degradation in biocathode microbial fuel cells (MFCs). Bacterial communities acclimated with either 4-chlorophenol (4-CP) or 2,4-dichlorophenol (2,4-DCP) under anaerobiosis can degrade the respective substrates more efficiently than the facultative aerobic bacterial communities. The anaerobic bacterial communities well developed with 2,4-DCP were then adapted to 2,4,6-trichlorophenol (2,4,6-TCP) and successfully stimulated for enhanced 2,4,6-TCP degradation and power generation. A 2,4,6-TCP degradation rate of 0.10 mol/m³/d and a maximum power density of 2.6 W/m³ (11.7 A/m³) were achieved, 138 and 13 % improvements, respectively compared to the controls with no stimulation. Bacterial communities developed with the specific CPs under anaerobic/aerobic conditions as well as the stimulated biofilm shared some dominant genera and also exhibited great differences. These results provide the most convincing evidence to date that anaerobic/aerobic conditions affected CPs degradation with power generation from the biocathode systems, and using deliberate substrates can stimulate the microbial consortia and be potentially feasible for the selection of an appropriate microbial community for the target substrate (e.g. 2,4,6-TCP) degradation in the biocathode MFCs.     

The algal microflora of the salt works of Tarquinia (Italy)

The microflora and benthic microbial mats of the salt works of Tarquinia (Italy) are described. From 144 samples, 111 species and varieties of microalgae were identified. Diatoms prevailed up to 110. S. They were substituted by Cyanophyta at higher salinities. The microbial mats consisted of Cyanophyta, Beggiatoa spp., purple sulphur bacteria, and diatoms. Information on hypertonic resistance in some euryhaline species is given.     

Microbial Communities and Fecal Indicator Bacteria Associated with Cladophora Mats on Beach Sites along Lake Michigan Shores

A high biomasses of Cladophora, a filamentous green alga, is found mainly during the summer along the shores of Lake Michigan. In this study, the abundance and persistence of the fecal indicator bacterium Escherichia coli and sulfate-reducing bacteria (SRB) on Cladophora mats collected at Lake Michigan beaches were evaluated using both culture-based and molecular analyses. Additionally, 16S rRNA gene cloning and sequencing were used to examine the bacterial community composition. Overall, E. coli was detected in all 63 samples obtained from 11 sites, and the average levels at most beaches ranged from 2,700 CFU/100 g (wet weight) of Cladophora to 7,500 CFU/100 g of Cladophora. However, three beaches were found to have site average E. coli densities of 12,800, 21,130, and 27,950 CFU/100 g of Cladophora. The E. coli levels in the lake water collected at the same time from these three sites were less than the recommended U.S. Environmental Protection Agency limit, 235 CFU/100 ml. E. coli also persisted on Cladophora mats in microcosms at room temperature for more than 7 days, and in some experiments it persisted for as long as 28 days. The SRB densities on Cladophora mats were relatively high, ranging from 4.4 × 10⁶ cells/g (6.64 log CFU/g) to 5.73 × 10⁶ cells/g (6.76 log CFU/g) and accounting for between 20% and 27% of the total bacterial counts. Partial sequences of the 16S rRNA gene clones revealed a phylogenetically diverse community, in which the Cytophaga-Flavobacterium-Bacteroides cluster and the low-G+C-content gram-positive bacteria were the dominant organisms, accounting for 40% and 12.8%, respectively, of the total clone library. These results further reveal the potential public health and ecological significance of Cladophora mats that are commonly found along the shoreline of Lake Michigan, especially with regard to the potential to harbor microorganisms associated with fecal pollution and odor-causing bacteria.     

Biogeochemistry of microbial mats under Precambrian environmental conditions: a modelling study

Microbial mats have arguably been the most important ecosystem on Earth over its 3.5 Gyr inhabitation. Mats have persisted as consortia for billions of years and occupy some of Earth's most hostile environments. With rare exceptions (e.g. microbial mats developed on geothermal springs at Yellowstone National Park, USA), today's mats do not exist under conditions analogous to Precambrian habitats with substantially lower oxygen and sulphate concentrations. This study uses a numerical model of a microbial mat to investigate how mat composition in the past might have differed from modern mats. We present a numerical model of mat biogeochemistry that simulates the growth of cyanobacteria (CYA), colourless sulphur bacteria (CSB), and purple sulphur bacteria (PSB), with sulphate‐reducing bacteria (SRB) and heterotrophic bacteria represented by parameterized sulphate reduction rates and heterotrophic consumption rates, respectively. Variations in the availability of light, oxygen, sulphide, and sulphate at the upper boundary of the mat are the driving forces in the model. Mats with remarkably similar biomass and chemical profiles develop in models under oxygen boundary conditions ranging from 2.5 × 10^?13 to 0.25 mm and sulphate boundary concentrations ranging from 0.29 to 29 mm , designed to simulate various environments from Archean to modern. The modelled mats show little sensitivity to oxygen boundary conditions because, independent of the overlying oxygen concentrations, cyanobacterial photosynthesis creates similar O₂ concentrations of 0.45–0.65 mm in the upper reaches of the mat during the photoperiod. Varying sulphate boundary conditions have more effect on the biological composition of the mat. Sulphide generated from sulphate reduction controls the magnitude and distribution of the PSB population, and plays a part in the distribution of CSB. CSB are the most sensitive species to environmental change, varying with oxygen and sulphide.     

Molecular phylogenetic and chemical analyses of the microbial mats in deep-sea cold seep sediments at the northeastern Japan Sea

Microbial communities inhabiting deep-sea cold seep sediments at the northeastern Japan Sea were characterized by molecular phylogenetic and chemical analyses. White patchy microbial mats were observed along the fault offshore the Hokkaido Island and sediment samples were collected from two stations at the southern foot of the Shiribeshi seamount (M1 site at a depth of 2,961 m on the active fault) and off the Motta Cape site (M2 site at a depth of 3,064 m off the active fault). The phylogenetic and terminal-restriction fragment polymorphism analyses of PCR-amplified 16S rRNA genes revealed that microbial community structures were different between two sampling stations. The members of ANME-2 archaea and diverse bacterial components including sulfate reducers within Deltaproteobacteria were detected from M1 site, indicating the occurrence of biologically mediated anaerobic oxidation of methane, while microbial community at M2 site was predominantly composed of members of Marine Crenarchaeota group I, sulfate reducers of Deltaproteobacteria, and sulfur oxidizers of Epsilonproteobacteria. Chemical analyses of seawater above microbial mats suggested that concentrations of sulfate and methane at M1 site were largely decreased relative to those at M2 site and carbon isotopic composition of methane at M1 site shifted heavier (¹³C-enriched), the results of which are consistent with molecular analyses. These results suggest that the mat microbial communities in deep-sea cold seep sediments at the northeastern Japan Sea are significantly responsible for sulfur and carbon circulations and the geological activity associated with plate movements serves unique microbial habitats in deep-sea environments.     

The removal of pyritic sulphur from coal by Leptospirillum-like bacteria

Summary The microbial oxidation of pyritic sulphur was studied in a 4.5-l airlift fermentor at pH 1.5 and 100 g/l pulp density. By microbial leaching with Leptospirillum-like bacteria 85% of the pyritic sulphur was removed within 40 days; 30% of the removed pyrite was oxidized to elemental sulphur, the rest being transformed to soluble sulphate. Accumulation of elemental sulphur could be avoided by using a mixed culture of Leptospirillum-like bacteria and Thiobacillus ferrooxidans. Apart from oxidation of elemental sulphur neither the pure nor the mixed culture showed a significant difference as to removal of pyrite.     

Effect of storage conditions on detection of mycoplasma in biopharmaceutical products

Mycoplasma contamination affects many different aspects of cell culturing, resulting in unreliable experimental results and potentially harmful biological products. Therefore, the specificity, sensitivity, and reliability of detecting mycoplasma contamination are important aspects of quality control in biotechnological products. In this study, Mycoplasma hyorhinis was adopted as a model strain to evaluate the effects of storage on the viability of Mycoplasma species in cell culture samples. Medium X was compared with conventional media 243 and 988 for the ability to detect M. hyorhinis. The 10¹ CFU/ml of M. hyorhinis was inoculated into medium X prepared using the same lots of components and preserved for 7 d, 1 mo, and 2 mo. M. hyorhinis grew readily and typically on agar plates prepared within 1 mo. The viable mycoplasmas in samples containing different initial titers (10¹ and 10⁶ CFU/ml) after storage at 4° C and −30° C were analyzed. During storage, viable organisms were found with little or no reduction in titers after storage for 8 wk at −30° C under aerobic and anaerobic conditions. A reduction in titers of 3 log10 occurred after 4 wk storage for high-dose cultures (10⁶ CFU/ml) at 4° C. The titers of viable organisms were diminished over 8 wk at 4° C under aerobic and anaerobic conditions.     

Effects of antimicrobial agents fed to chickens on some gram-negative enteric bacilli.

Total and antimicrobial agent-resistant aerobic and facultative anaerobic gram-negative enteric bacilli in fecal samples of broiler chickens fed growth-promotional levels of antimicrobial agents were determined quantitatively. Two 8-week studies were conducted utilizing groups of chickens fed antimicrobial-supplemented rations; the second study involved feed "pasteurization" as a means of minimizing colonization from the feed. Dilution/spread-plating/replica-plating techniques on selective media were used to obtain counts of total organisms and those resistant to tetracycline, chloramphenicol, streptomycin, ampicillin, or kanamycin. The predominant aerobic and facultative anaerobic gram-negative organism was Escherichia coli, which was detected in all samples at levels ranging from 10(5) to over 10(10) CFU/g of feces. Less common were Proteus mirabilis, Klebsiella pneumoniae, and Pseudomonas sp., which varied in occurrence and levels from group to group (range, less than 10(3) to 10(8) CFU/g). Resistance to all antimicrobials (except chloramphenicol in E. coli) was commonly observed at incidences exceeding 10(3) CFU/g in the total populations. Colonization of the chickens' intestinal tracts by susceptible and resistant strains of E. coli, K. pneumoniae, and Pseudomonas sp. appeared to result from their presence in the environment of the newly hatched chickens. Ration pasteurization did affect P. mirabilis, which appeared to colonize from the feed. The results suggest that colonization by, and proliferation of, antimicrobial-resistant aerobic and facultative anaerobic gram-negative enteric bacilli in chicken intestinal tracts may be less dependent on selection through antimicrobial supplementation of the ration than on their prevalence in environments from which they can colonize newborns.     

Effects of antimicrobial agents fed to chickens on some gram-negative enteric bacilli

Total and antimicrobial agent-resistant aerobic and facultative anaerobic gram-negative enteric bacilli in fecal samples of broiler chickens fed growth-promotional levels of antimicrobial agents were determined quantitatively. Two 8-week studies were conducted utilizing groups of chickens fed antimicrobial-supplemented rations; the second study involved feed "pasteurization" as a means of minimizing colonization from the feed. Dilution/spread-plating/replica-plating techniques on selective media were used to obtain counts of total organisms and those resistant to tetracycline, chloramphenicol, streptomycin, ampicillin, or kanamycin. The predominant aerobic and facultative anaerobic gram-negative organism was Escherichia coli, which was detected in all samples at levels ranging from 10(5) to over 10(10) CFU/g of feces. Less common were Proteus mirabilis, Klebsiella pneumoniae, and Pseudomonas sp., which varied in occurrence and levels from group to group (range, less than 10(3) to 10(8) CFU/g). Resistance to all antimicrobials (except chloramphenicol in E. coli) was commonly observed at incidences exceeding 10(3) CFU/g in the total populations. Colonization of the chickens' intestinal tracts by susceptible and resistant strains of E. coli, K. pneumoniae, and Pseudomonas sp. appeared to result from their presence in the environment of the newly hatched chickens. Ration pasteurization did affect P. mirabilis, which appeared to colonize from the feed. The results suggest that colonization by, and proliferation of, antimicrobial-resistant aerobic and facultative anaerobic gram-negative enteric bacilli in chicken intestinal tracts may be less dependent on selection through antimicrobial supplementation of the ration than on their prevalence in environments from which they can colonize newborns.