Single disc method for minimum inhibitory concentration (MIC) determination.

Salmonella enterica grown on solid medium containing iron, thiosulfate and 100 mM hexoses and amino acids underwent cell surface differentiation involving increased flagellation (electrophoretic isotypes 60, 54 and 50 kDa), conversion from rough to smooth lipopolysaccharide, and assembly of a matrix that penetrated 1.4% agar. Flagellation was also induced in the avian pathogen S. enterica var Pullorum, which is diagnostically defined as aflagellate. Induction correlated closely with a simple colonial color change when Hektoen Enteric agar was used as the basal growth medium. Group D1 egg-contaminating Salmonella grown under inducing conditions deviated from their expected H-antigen immunoreactivity, suggesting possible consequences for the interpretation of the Kauffman-White identification scheme.

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PubMed1997

Critical volume of rat cortex and extracellular threshold concentration for a pentylenetetrazol-induced epileptic focus.

The initiation of focal interictal epileptiform activity (FIEA) has been shown to depend on the activation of a sufficiently large volume of brain tissue. We estimated the size of this 'critical volume' for the convulsant pentylenetetrazol (PTZ) by analyzing the diffusion following its microinjection into rat motor cortex. PTZ concentration was monitored 100-200 microm away from the injection site with a PTZ-sensitive microelectrode. Diffusion analysis in 0.3% agar yielded the free diffusion coefficient D (8.50 +/- 0.15 X 10(-6) cm2 x s(-1) at 37 degrees C, median +/- S.E.M.). In brain tissue, diffusion was modified by extracellular volume fraction (alpha), tortuosity (lambda = (D/ADC)1/2; ADC = apparent diffusion coefficient) and non-specific uptake (k'). Using a value of 0.2 for alpha from previous studies, we found values of lambda = 1.61 +/- 0.01, k' = 3.37 +/- 0.15 X 10(-3) s(-1) and an injected volume U of 5.16 +/- 0.45 X 10(-10) l for pulses without FIEA, and lambda = 1.95 +/- 0.06, k' = 6.24 +/- 1.73 X 10(-3) s(-1) and U = 7.40 +/- 0.66 X 10(-10) l for pulses with FIEA. From the calculated concentration distribution of PTZ during FIEA we estimated a threshold concentration of about 1.77 mM PTZ and a volume with a radius of about 219 microm in which this concentration had to be exceeded. Since this critical volume was comparable in size to foci elicited by penicillin or electric stimuli in previous studies, it is concluded that it is determined by intrinsic tissue properties rather than by the convulsive agent being used.

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PubMed1997

Optimisation of methodology for enumeration of xerophilic yeasts from foods.

Xerophilic yeasts grow in intermediate moisture foods (aw, 0.65-0.85) such as sugar syrups, fruit concentrates, jams and brines. Non-osmophilic yeasts are enumerated by diluting in 0.1% peptone and then plated onto media such as malt extract or glucose yeast extract agar. In the presence of moulds the yeasts are enumerated in dichloran rose bengal chloramphenicol agar (DRBC). These procedures were demonstrated to be unsatisfactory for the enumeration of xerophilic yeasts in low aw foods. Investigations using pure cultures of xerophilic yeasts as well as naturally contaminated apple juice concentrates and glacé cherries have shown that a reduced aw diluent, in particular 30% w/w glycerol in combination with tryptone 10% glucose yeast extract agar (TGY) optimises the recovery of the yeasts, especially sublethally injured cells. The inclusion of sodium chloride in either the diluents or the culture media was not necessary to optimise the recovery of D. hansenii growing in 20% sodium chloride broths.

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