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Detection and Susceptibility Testing of Hypermutable Pseudomonas aeruginosa Strains with the Etest and Disk Diffusion. Maria D. Maciá, 2004.Resistance development in Pseudomonas aeruginosa from chronically colonized cystic fibrosis (CF) patients has been linked to the presence of a high proportion of mismatch repair-deficient hypermutable strains . The detection of hypermutable strains by microbiology laboratories may be useful for establishing adequate antimicrobial therapies . In this work, we find that the Etest and disk diffusion can be used as simple methods for the detection and susceptibility testing of hypermutable P . aeruginosa isolates . Strain PAO1 and its hypermutable derivative strain PAO  mutS were used to standardize the procedure, which was tested with 35 P . aeruginosa isolates from 21 CF patients . Mutation frequencies were estimated by standard methods, and 29% of the isolates were found to be hypermutable . MICs and inhibition zone diameters were determined for ceftazidime, imipenem, meropenem, ciprofloxacin, and tobramycin by using Etest strips and conventional disks, respectively . The presence (or absence) of resistant mutant subpopulations, as well as their relative numbers and the highest MICs for them (or smallest inhibition zone diameters), was recorded . The presence of resistant mutant subpopulations within the inhibition zones of three or more antibiotics clearly identified the strains as hypermutable (they were present in 10 of 10 hypermutable strains and 0 of 25 nonhypermutable strains) with both methods . Additionally, these methods allowed us to differentiate between dual effects of hypermutation in antibiotic resistance, namely, that (i) hypermutable isolates were substantially more resistant than nonhypermutable isolates and that (ii) the resistance of hypermutable isolates was dramatically increased by the presence of resistant mutant subpopulations . This differentiation may be relevant for the design of adequate treatments, since the second effect, in contrast to the first, may be overcome by antibiotic combinations .
Selection and Identification of a Listeria monocytogenes Target Strain for Pulsed Electric Field Process Optimization. Beatrice H. Lado, 2003.Nine Listeria monocytogenes strains were treated individually with a continuous pulsed electric field (PEF) apparatus, and their sensitivities to the treatment were compared at 25 kV/cm . When cell suspensions of these strains in 0.1% NaCl (pH 7.0) were treated at 23°C for 144 µs, inactivation ranged from 0.7 to 3.7 log10 CFU/ml . Inactivation by 72-µs PEF treatments at 37°C ranged from 0.3 to 2.5 log10 CFU/ml . L . monocytogenes OSY-8578 was substantially more resistant than other strains when cells were PEF treated in 0.1% NaCl, whereas Scott A was one of the most sensitive strains . The superiority of OSY-8578's resistance to that of Scott A was confirmed in 50% diluted acid whey (pH 4.2) . Changes in sensitivity to PEF during phases of growth were minimal in OSY-8578 and substantial in Scott A . Use of L . monocytogenes OSY-8578, therefore, is recommended in studies to optimize PEF processes that target L . monocytogenes . The nine L . monocytogenes strains were genotyped with pulsed-field gel electrophoresis (PFGE) and arbitrarily primed PCR (AP-PCR) techniques . These strains were better differentiated with PFGE than with AP-PCR . The target strain (OSY-8578) was characterized by both molecular typing techniques, but resistance to PEF, in general, was not associated with a particular genotype group . Chlorination and Cleavage of Lignin Structures by Fungal Chloroperoxidases. Patricia Ortiz-Bermúdez, 2003.Two fungal chloroperoxidases (CPOs), the heme enzyme from Caldariomyces fumago and the vanadium enzyme from Curvularia inaequalis, chlorinated 1-(4-ethoxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)-1,3-dihydroxypropane, a dimeric model compound that represents the major nonphenolic structure in lignin . Both enzymes also cleaved this dimer to give 1-chloro-4-ethoxy-3-methoxybenzene and 1,2-dichloro-4-ethoxy-5-methoxybenzene, and they depolymerized a synthetic guaiacyl lignin . Since fungal CPOs occur in soils and the fungi that produce them are common inhabitants of plant debris, CPOs may have roles in the natural production of high-molecular-weight chloroaromatics and in lignin breakdown .
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