A simple mathematical model that describes the growth of the area and the number of total and viable cells in yeast colonies.

UNLABELLED: We propose a model, based on the Gompertz equation, to describe the growth of yeasts colonies on agar medium. This model presents several advantages: (i) one equation describes the colony growth, which previously needed two separate ones (linear increase of radius and of the squared radius); (ii) a similar equation can be applied to total and viable cells, colony area or colony radius, because the number of total cells in mature colonies is proportional to their area; and (iii) its parameters estimate the cell yield, the cell concentration that triggers growth limitation and the effect of this limitation on the specific growth rate. To elaborate the model, area, total and viable cells of 600 colonies of Saccharomyces cerevisiae, Debaryomyces fabryi, Zygosaccharomyces rouxii and Rhodotorula glutinis have been measured. With low inocula, viable cells showed an initial short exponential phase when colonies were not visible. This phase was shortened with higher inocula. In visible or mature colonies, cell growth displayed Gompertz-type kinetics. It was concluded that the cells growth in colonies is similar to liquid cultures only during the first hours, the rest of the time they grow, with near-zero specific growth rates, at least for 3 weeks. SIGNIFICANCE AND IMPACT OF THE STUDY: Mathematical models used to predict microbial growth are based on liquid cultures data. Models describing growth on solid surfaces, highlighting the differences with liquids cultures, are scarce. In this work, we have demonstrated that a single Gompertz equation describes accurately the increase of the yeast colonies, up to the point where they reach their maximum size. The model can be used to quantify the differences in growth kinetics between solid and liquid media. Moreover, as all its parameters have biological meaning, it could be used to build secondary models predicting yeast growth on solid surfaces under several environmental conditions.

Performance of mycological media in enumerating desiccated food spoilage yeasts: an interlaboratory study.

Dichloran 18% glycerol agar (DG18) was originally formulated to enumerate nonfastidious xerophilic moulds in foods containing rapidly growing Eurotium species. Some laboratories are now using DG18 as a general purpose medium for enumerating yeasts and moulds, although its performance in recovering yeasts from dry foods has not been evaluated. An interlaboratory study compared DG18 with dichloran rose bengal chloramphenicol agar (DRBC), plate count agar supplemented with chloramphenicol (PCAC), tryptone glucose yeast extract chloramphenicol agar (TGYC), acidified potato dextrose agar (APDA), and orange serum agar (OSA) for their suitability to enumerate 14 species of lyophilized yeasts. The coefficient of variation for among-laboratories repeatability within yeast was 1.39% and reproducibility of counts among laboratories was 7.1%. The order of performance of media for recovering yeasts was TGYC > PCAC = OSA > APDA > DRBC > DG 18. A second study was done to determine the combined effects of storage time and temperature on viability of yeasts and suitability of media for recovery. Higher viability was retained at -18 degrees C than at 5 degrees C or 25 degrees C for up to 42 weeks, although the difference in mean counts of yeasts stored at -18 degrees C and 25 degrees C was only 0.78 log10 cfu/ml of rehydrated suspension. TGYC was equal to PCAC and superior to the other four media in recovering yeasts stored at -18 degrees C, 5 degrees C, or 25 degrees C for up to 42 weeks. Results from both the interlaboratory study and the storage study support the use of TGYC for enumerating desiccated yeasts. DG18 is not recommended as a general purpose medium for recovering yeasts from a desiccated condition.

A chromogenic medium for the detection of yeasts with beta-galactosidase and beta-glucosidase activities from intermediate moisture foods.

A selective and differential solid medium for the specific detection of some common yeasts frequently causing spoilage in intermediate moisture foods is described. The principle of the method is based on the detection of two enzymes, beta-glucosidase and beta-galactosidase, using the chromogenic substrates salmon-Gluc and X-Gal. Over 140 yeasts and bacteria were tested, and Debaryomyces hansenii and Kluyveromyces marxianus strains produced salmon and dark blue colonies, respectively, thus permitting their clear discrimination from other yeasts common in intermediate moisture foods. The medium was very satisfactory when intermediate moisture foods were tested.

A differential medium for the isolation of Kluyveromyces marxianus and Kluyveromyces lactis from dairy products.

A selective and differential solid medium, called Kluyveromyces Differential Medium (KDM), is described for the isolation of Kluyveromyces marxianus and K. lactis from dairy products. Its discriminative potential is based on the detection of the enzyme beta-galactosidase, in the absence of lactose. Of the more than 95 strains tested, including yeasts, bacteria, and filamentous fungus, only the strains of K. marxianus and K. lactis produced blue colonies on the medium due to the presence of X-Gal/ IPTG. The bacterial strains were not able to grow in KDM. On this basis, the medium was very satisfactory when testing naturally or experimentally contaminated dairy food products. When quality assessment tests were performed, optimal values of productivity (growth and color) and selectivity were obtained for K. marxianus and K. lactis.

Distribution of oxidizing bacterial activities and characterization of bioleaching-related microorganisms in a uranium mineral heap.

The occurrence and activity of bioleaching-related microorganisms are highest at 0.25 m under the surface of a uranium mineral heap, and decrease at points deeper than 1 m inside the heap. Thiobacillus ferrooxidans, Th. acidophilus, Th. thiooxidans and Leptospirillum ferrooxidans have been found and isolated from different sites inside the heap. Other mesophilic iron- or sulphur-oxidizing bacteria and some moderate thermophile have also been isolated from deep (1 m and 4 m) sites and characterized. Several fungi and some yeasts are present in this bioleaching habitat.

Comparison of quantitative analytical methods in evaluating acidophilic iron- and sulphur-oxidizing microorganisms liquid cultures.

Different quantitative methods to evaluate cell concentrations in acidophilic iron- and sulphur-oxidizing microorganisms liquid cultures have been compared with the "most probable number of viable germs" procedure. Plating and colony counting on recently developed solid media give good results when used with pure bacterial cultures, but plating efficiency decreases with natural mixed cultures.