PubMed · 6690992
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A H Bretag. What is the solution?. https://doi.org/10.1038/307116a0
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Unlike most other bacteria, many Listeria strains do not grow well in the minimal media described so far in the literature. Among the minimal media tested, a chemically defined medium modified from that of Premaratne and co-workers was found to support the best growth of Listeria spp. The promoting effect was due to the incorporation of several indispensable vitamins and growth factors.
The combined effect of medium water activity (aw) modified atmosphere packaging (MAP) on the conidial germination of Aspergillus niger, Eurotium amstelodami, Penicillium chrysogenum and Fusarium oxysporum was studied by applying response surface methodology (RSM) for 45 days incubation at 20, 30 and 40 degrees C. Oxytetracycline glucose yeast extract (OGYE) agar media were prepared over a wide range of aw (0.80-0.95), controlled by glucose. The headspace atmosphere composition varied from 0-20% for O2 and from 0-80% for CO2 with N2 as balance in the ratio medium/gas mixture of 1/3 (v/v). The results of this experiment were confirmed by repeating some combinations (aw-MAP) in the zone where the inhibition was effective. In contrast to A. niger which germinated and grew better at 30 and 40 degrees C, P. chrysogenum and F. oxysporum were capable of germinating and growing at 20 degrees C but not at 40 degrees C. E. amstelodami was less influenced by the incubation temperature than were the other species. Under anaerobic atmospheres, germination and growth of all tested species were completely inhibited at low aw, while at high aw because of residual O2, conidia germinated but failed to grow. At any aw value between 0.88 and 0.92, conidia failed to germinate when 100% CO2 was applied. Similar results were obtained for E. amstelodami and F. oxysporum in 80% CO2-20% N2 and 60% CO2-40%N2. Under aerobic conditions, at 5% O2 germination and growth occurred only at high aw, while 10 or 20% O2 combined with either 80 or 60% CO2 conidial germination and mould growth were only delayed comparing to the control (air).
Ashbya gossypii can grow on triacyglycerol as carbon source. A degradation rate of 0.05 g x g-1 mycelial dry mass x h-1 was detected for soybean oil. Although this rate was within the sensitivity range of lipase assays no activity was detectable. On the other hand, extracellular lipase activity could be visualized by clearance halos round the growing mycelium when trioleoylglycerol was emulsified as the sole carbon source in agar plates. Variation of the culture conditions revealed that reduced shaking speed and decreased fat content in the medium led to detectable amounts of lipase in the supernatant of flask cultures. A maximal activity of 800 U x l-1 was obtained after 32 h of cultivation in flasks containing 1% yeast extract and incubated at 60 rpm. Because of its pI of 9.0, the enzyme could be purified in a single step by preparative isoelectric focusing. It appeared as a homogeneous protein in analytical isoelectric focusing and SDS/PAGE (M 35,000). The lipase was inactivated within minutes in stirred gas/water, trioleoylglycerol/water or oleic acid/water mixtures. These effects suggested an interface inactivation. This idea was supported by a stability modulation observed with the surfactant Pluronic F-68. Inactivation by oleic acid led to an aggregation of the lipase shown by gel filtration. Growth experiments performed under lipase-stabilizing conditions revealed a negative influence of glucose, glycerol or oleic acid on detectable lipase activity, probably due to a regulation of lipase formation. Inactivation and regulation thus explained the lack of detectable lipase activity in cultures of A. gossypii growing on triacylglycerol.