Agar Medium for Differential Enumeration of Lactic Streptococci

Airborne transmission of biological agents within the indoor built environment: a multidisciplinary review

The nature and airborne dispersion of the underestimated biological agents, monitoring, analysis and transmission among the human occupants into building environment is a major challenge of today. Those agents play a crucial role in ensuring comfortable, healthy and risk-free conditions into indoor working and leaving spaces. It is known that ventilation systems influence strongly the transmission of indoor air pollutants, with scarce information although to have been reported for biological agents until 2019. The biological agents' source release and the trajectory of airborne transmission are both important in terms of optimising the design of the heating, ventilation and air conditioning systems of the future. In addition, modelling via computational fluid dynamics (CFD) will become a more valuable tool in foreseeing risks and tackle hazards when pollutants and biological agents released into closed spaces. Promising results on the prediction of their dispersion routes and concentration levels, as well as the selection of the appropriate ventilation strategy, provide crucial information on risk minimisation of the airborne transmission among humans. Under this context, the present multidisciplinary review considers four interrelated aspects of the dispersion of biological agents in closed spaces, (a) the nature and airborne transmission route of the examined agents, (b) the biological origin and health effects of the major microbial pathogens on the human respiratory system, (c) the role of heating, ventilation and air-conditioning systems in the airborne transmission and (d) the associated computer modelling approaches. This adopted methodology allows the discussion of the existing findings, on-going research, identification of the main research gaps and future directions from a multidisciplinary point of view which will be helpful for substantial innovations in the field.

Comparison of growth and survival of single strains of Lactococcus lactis and Lactococcus cremoris during Cheddar cheese manufacture

Traditionally, starter cultures for Cheddar cheese are combinations of Lactococcus lactis and Lactococcus cremoris. Our goal was to compare growth and survival of individual strains during cheesemaking, and after salting and pressing. Cultures used were 2 strains of L. lactis (SSM 7605, SSM 7436) and 2 strains of L. cremoris (SSM 7136, SSM 7661). A standardized Cheddar cheese make procedure was used that included a 38°C cook temperature and salting levels of 2.0, 2.4, 2.8, 3.2, and 3.6% from which were selected cheeses with salt-in-moisture levels of 3.5, 4.5, and 5.5%. Vats of cheese were made using each strain on its own as biological duplicates on different days. Starter culture numbers were enumerated by plate counting during cheesemaking and after 6 d storage at 6°C. Flow cytometry with fluorescent staining by SYBR Green and propidium iodide was used to determine the number of live and dead cells in cheese at the different salt levels. Differences in cheese make times were strain dependent rather than species dependent. Even with correction for average culture chain length, cheeses made using L. lactis strains contained ∼4 times (∼0.6 log) more bacterial cells than those made using L. cremoris strains. Growth of the strains used in this study was not influenced by the amount of salt added to the curd. The higher pH of cheeses with higher salting levels was attributed to those cheeses having a lower moisture content. Based on flow cytometry, ∼5% of the total starter culture cells in the cheese were dead after 6 d of storage. Another 3 to 19% of the cells were designated as being live, but semipermeable, with L. cremoris strains having the higher number of semipermeable cells.

Functional implications of the microbial community structure of undefined mesophilic starter cultures

This review describes the recent advances made in the studies of the microbial community of complex and undefined cheese starter cultures. We report on work related to the composition of the cultures at the level of genetic lineages, on the presence and activity of bacteriophages and on the population dynamics during cheese making and during starter culture propagation. Furthermore, the link between starter composition and starter functionality will be discussed. Finally, recent advances in predictive metabolic modelling of the multi-strain cultures will be discussed in the context of microbe-microbe interactions.

Identification of some lactic acid bacteria from two Zimbabwean fermented milk products

Ten predominant lactic acid bacterial isolates from traditionally fermented milk and four isolates from an industrially fermented milk, Lacto, together with 14 reference strains ofLactobacillus and three ofLactococcus were examined for 32 characteristics. Data were analysed using the simple matching coefficient and clustering was by unweighted pair group average linkage. All the isolated from traditionally fermented milk belonged to the genusLactobacillus. Seven isolates could be identified as belonging toL. helveticus, L. plantarum, L. delbrueckii subsp.lactis, L. casei subsp.casei andL. casel subsp.pseudoplantarum. Three of the isolates could only be identified as either betabacteria or streptobacteria. The four isolates from Lacto were identified asLactococcus lactis. However, they could not be identified to subspecies level.

Characterization of plasmid-encoded citrate permease (citP) genes from Leuconostoc species reveals high sequence conservation with the Lactococcus lactis citP gene

The citrate permease determinant (citP) in several Leuconostoc strains was demonstrated to be plasmid encoded by curing experiments and hybridization studies with a DNA fragment containing the citP gene from Lactococcus lactis subsp. lactis biovar diacetylactis NCDO176. Cloning and nucleotide sequence analysis of Leuconostoc lactis NZ6070 citP revealed almost complete identity to lactococcal citP.

Development and use of a selective medium for isolation of Leuconostoc spp. from vegetables and dairy products

A selective medium (LUSM medium) for the isolation of Leuconostoc spp. was developed. This medium contained 1.0% glucose, 1.0% Bacto Peptone (Difco), 0.5% yeast extract (BBL), 0.5% meat extract (Difco), 0.25% gelatin (Difco), 0.5% calcium lactate, 0.05% sorbic acid, 75 ppm of sodium azide (Sigma), 0.25% sodium acetate, 0.1% (vol/vol) Tween 80, 15% tomato juice, 30 micrograms of vancomycin (Sigma) per ml, 0.20 microgram of tetracycline (Serva) per ml, 0.5 mg of cysteine hydrochloride per ml, and 1.5% agar (Difco). LUSM medium was used successfully for isolation and enumeration of Leuconostoc spp. in dairy products and vegetables. Of 116 colony isolates obtained from fresh raw milk, curdled milk, or various vegetables, 115 were identified as members of the genus Leuconostoc. A total of 89 of these isolates were identified to species; 13.5% of the isolates were Leuconostoc cremoris, 7.9% were Leuconostoc mesenteroides subsp. mesenteroides, 11.2% were Leuconostoc mesenteroides subsp. dextranicum, 16.9% were Leuconostoc mesenteroides subsp. paramesenteroides, 10.1% were leuconostoc lactis, and 40.4% were Leuconostoc oenos. When we compared the counts obtained for two Leuconostoc strains, Leuconostoc dextranicum 181 and L. cremoris JLL8, on MRS agar and LUSM medium, we found no significant difference between the values obtained on the two media.

Study of the Citrate Metabolism of Lactococcus lactis subsp. lactis Biovar Diacetylactis by Means of 13 C Nuclear Magnetic Resonance

The metabolic fate of citrate and pyruvate in four strains of Lactococcus lactis subsp. lactis biovar diacetylactis has been studied by means of 13 C nuclear magnetic resonance, using as a substrate either [3- 13 C]pyruvic acid or custom-synthesized citric acid that is 13 C labeled either at carbons 2 and 4 or at carbon 3. The fermentations were carried out batchwise in modified M17 broth. For the actual conversions of the 13 C-labeled substrates, cells at the end of their logarithmic growth phase were used to minimize the conversion to lactic acid. A mass balance of the main citric acid metabolites was obtained; the four strains produced from 50 to 70% (on a molar basis) lactic acid from either citrate or pyruvate. The remaining 50 to 30% was converted mainly to either α-acetolactic acid (for one strain) or acetoin (for the other three strains). One of the strains produced an exceptionally high concentration of the diacetyl precursor α-acetolactic acid. Another strain (SDC6) also produced α-acetolactic acid, but this was decarboxylated to acetoin at a high rate. The 13 C nuclear magnetic resonance method confirmed that the biosynthesis of α-acetolactic acid occurs via condensation of pyruvate and “active” acetaldehyde. Diacetyl was not found as a direct metabolite of citrate or pyruvate metabolism.

The use of mesophilic cultures in the dairy industry

The use of mesophilic starter cultures, containing group N Streptococcus and Leuconostoc species, in the dairy industry is examined. Bacteriophage attack is identified as the main cause of culture inhibition and criteria used to select stable mixed-strain starter cultures and phage-insensitive defined strains are established. The key aspects of culture propagation and storage are highlighted, as well as bulk-culture protection based on physical exclusion of phage and the use of phage-inhibitory media. Developments in the application of starter culture systems in different countries are examined and show that significant process control and elimination of phage problems can be achieved.

Improved Medium for Detection of Citrate-Fermenting Streptococcus lactis subsp. diacetylactis

A modified medium which distinguished between citrate-fermenting and non-citrate-fermenting species of lactic streptococci within 48 h was developed. In addition, the occurrence of citrate-negative variants in citrate-positive populations of Streptococcus lactis subsp. diacetylactis could be detected.