Effects of storage temperature on bacterial growth rates and community structure in fresh retail sushi.

AIMS: This study was conducted to assess the effects of different storage temperatures (4-20°C), on bacterial concentrations, growth rates and community structure in fresh retail sushi, a popular retail product with a claimed shelf life of 2-3 days. METHODS AND RESULTS: The maximum specific growth rate based on aerobic plate count (APC) at 4°C was 0·06 h-1 and displayed a sixfold increase (0·37 h-1 ) at 20°C. Refrigeration resulted in no growth of hydrogen sulphide (H2 S)-producing bacteria, but this group had the strongest temperature response. The bacterial community structure was determined by PCR/DGGE (denaturing gradient gel electrophoresis). Multivariate analysis based on Bray-Curtis similarities demonstrated that temperature alone was not the major determinant for the bacterial community structure. The total concentration of aerobic bacteria was the variable that most successfully explained the differences between the communities. The dominating organisms, detected by sequencing of DNA bands excised from the DGGE gel, were Brochothrix thermosphacta and genera of lactic acid bacteria (LAB). CONCLUSION: The relationship between growth rates and storage temperatures clearly demonstrates that these products are sensitive to deviations from optimal storage temperature, possibly resulting in loss of quality during shelf life. Regardless of the storage temperature, the bacterial communities converged towards a similar structure and density, but the storage temperature determined how fast the community reached its carrying capacity. SIGNIFICANCE AND IMPACT OF THE STUDY: Little information is available on the microbial composition of ready-to-eat food that are prepared with raw fish, subjected to contamination during handling, and susceptible to microbial growth during cold storage. Moreover, the data are a good first possibility to simulate growth of APC, H2 S-producing bacteria and LAB under different temperature scenarios that might occur during production, distribution or storage.

Microbial community dynamics and biogas production from manure fractions in sludge bed anaerobic digestion.

AIMS: To elucidate how granular sludge inoculum and particle-rich organic loading affect the structure of the microbial communities and process performance in upflow anaerobic sludge bed (UASB) reactors. METHODS AND RESULTS: We investigated four reactors run on dairy manure filtrate and four on pig manure supernatant for three months achieving similar methane yields. The reactors fed with less particle rich pig manure stabilized faster and had highest capacity. Microbial community dynamics analysed by a PCR/denaturing gradient gel electrophoresis approach showed that influent was a major determinant for the composition of the reactor communities. Comparisons of pre- and non-adapted inoculum in the reactors run on pig manure supernatant showed that the community structure of the nonadapted inoculum adapted in approximately two months. Microbiota variance partitioning analysis revealed that running time, organic loading rate and inoculum together explained 26 and 31% of the variance in bacterial and archaeal communities respectively. CONCLUSIONS: The microbial communities of UASBs adapted to the reactor conditions in treatment of particle rich manure fractions, obtaining high capacity, especially on pig manure supernatant. SIGNIFICANCE AND IMPACT OF THE STUDY: These findings provide relevant insight into the microbial community dynamics in startup and operation of sludge bed reactors for methane production from slurry fractions, a major potential source of biogas.

Evaluation of plate count methods for determination of maximum specific growth rate in mixed microbial communities, and its possible application for diversity assessment.

Two plate count methods are proposed for direct assessment of the maximum specific growth rate (microm) of bacteria in mixed communities. An estimate of microm of individual colonies is obtained by plating samples on an agar medium and determining either the time required to form macroscopically visible colonies, Tv (diameter < or =0.2 mm), or the linear radial growth rate, Kr, of single colonies. In accordance with theoretical models, a linear relationship was found between microm determined in liquid culture and 1/Tv, and between microm and Kr. Empirical relationships were established for these relationships. The time required to form a visible colony was 17 +/- 9 h longer for cells in the stationary growth phase, whereas the linear radial growth rate was not affected by the physiological state of the cells. The proposed plate count methods are simple and applicable for describing the community structure, and for estimating the frequency distribution of maximum specific growth rates in mixed communities. By using this frequency distribution it is possible to calculate diversity indexes and to assign a microbial community a position on an r/K-gradient.

Colonization of Vibrio pelagius and Aeromonas caviae in early developing turbot (Scophthalmus maximus L.) larvae.

Polyclonal antisera made in rabbits against whole washed cells of Vibrio pelagius and Aeromonas caviae were used for detection of these bacterial species in the rearing water and gastrointestinal tract of healthy turbot (Scophthalmus maximus) larvae exposed to V. pelagius and/or Aer. caviae. The results demonstrated that this method is suitable for detection of V. pelagius and Aer. caviae in water samples and larvae at population levels higher than 10(3) ml-1 and 10(3) larva-1. Populations of aerobic heterotrophic bacteria present in the gastrointestinal tract of turbot larvae, estimated using the dilution plate technique, increased from approximately 4 x 10(2) bacteria larva-1 on day 3 post-hatching to approximately 10(5) bacteria fish-1 16 days post-hatching. Sixteen days after hatching, Vibrio spp. accounted for approximately 3 x 10(4) cfu larva-1 exposed to V. pelagius on days 2, 5 and 8 post-hatching. However, only 10(3) of the Vibrio spp. belonged to V. pelagius. When larvae were exposed to Aer. caviae on day 2 post-hatching, the gut microbiota of 5-day old larvae was mainly colonized by Aeromonas spp. (10(4) larva-1), of which 9 x 10(3) belonged to Aer. caviae. Later in the experiment, at the time when high mortality occurred, 9 x 10(5) Aer. caviae were detected. Introduction of V. pelagius to the rearing water seemed to improve larval survival compared with fish exposed to Aer. caviae and with the control group. It was therefore concluded that it is beneficial with regard to larval survival to introduce bacteria (V. pelagius) to the rearing water.

Can phosphorus limitation inhibit dissolved organic carbon consumption in aquatic microbial food webs? A study of three food web structures in microcosms.

Microcosms with three different food web structures and phosphorus (P) limited growth medium were used to study the interactions between P and organic carbon (C) fractions in pelagic food webs. The cultures were run with low dilution to allow the biological processes to determine the outcome. A double isotope technique was used to follow the C and P compartments. In all systems the primary production was P limited. The measured P:C ratios and the observed accumulation of degradable dissolved organic carbon (DOC) indicated that the growth of heterotrophic bacteria was also P limited. The presence of neither algal grazers nor flagellates feeding on bacteria altered the limitation pattern. A net loss of P from the bacterial fraction was observed after the bloom. Different strategies for nutrient aquisition and growth are proposed as mechanisms enabling simultaneous P limitation of algae and bacteria, and a concomitant accumulation of degradable DOC. The ability of the algae to grow with low P:C ratio keeps the regeneration of P through grazers low enough to cause sustained P limitation of both algae and bacteria. The grazers were important producers of DOC when present. This implies that the usual assumption of carbon limited bacterial growth may lead to wrong conclusions regarding the dynamics of plankton communities and the DOC pool.