Electrotransformation of meat lactobacilli. Effect of several parameters on their efficiency of transformation

Modified electroporation protocol for Lactobacilli isolated from the chicken crop facilitates transformation and the use of a genetic tool

Isolates of Lactobacillus spp. from a collection of potentially probiotic strains isolated from the crops of broiler chickens were found to be non-electrotransformable using published techniques. One strain of Lactobacillus salivarius was shown to develop electrocompetence when an overnight culture was incubated in fresh medium. The effect was enhanced if glycine was incorporated into the fresh growth medium. When these modifications were applied to a number of other crop isolates of Lactobacillus spp., electrocompetence could be detected in approximately half the strains tested. Two temperature sensitive plasmid vectors that had been used for the genetic modification of other lactic acid bacteria were introduced into a crop strain of Lb. salivarius. Both showed temperature sensitivity at 42 degrees C and above but were relatively stable at 37 degrees C. The genetic tool harbouring an IS element allowed the delivery of the plasmid to multiple independent sites in the host chromosome. Harnessing such genetic tools will facilitate the future genetic analysis of the host bacterium.

Cloning and characterization of a prolinase gene (pepR) from Lactobacillus rhamnosus

A peptidase gene expressing L-proline-beta-naphthylamide-hydrolyzing activity was cloned from a gene library of Lactobacillus rhamnosus 1/6 isolated from cheese. Peptidase-expressing activity was localized in a 1.5-kb SacI fragment. A sequence analysis of the SacI fragment revealed the presence of one complete open reading frame (ORF1) that was 903 nucleotides long. The ORF1-encoded 34.2-kDa protein exhibited 68% identity with the PepR protein from Lactobacillus helveticus. Additional sequencing revealed the presence of another open reading frame (ORF2) following pepR; this open reading frame was 459 bp long. Northern (RNA) and primer extension analyses indicated that pepR is expressed both as a monocistronic transcriptional unit and as a dicistronic transcriptional unit with ORF2. Gene replacement was used to construct a PepR-negative strain of L. rhamnosus. PepR was shown to be the primary enzyme capable of hydrolyzing Pro-Leu in L. rhamnosus. However, the PepR-negative mutant did not differ from the wild type in its ability to grow and produce acid in milk. The cloned pepR expressed activity against dipeptides with N-terminal proline residues. Also, Met-Ala, Leu-Leu, and Leu-Gly-Gly and the chromogenic substrates L-leucine-beta-naphthylamide and L-phenylalanine-beta-naphthylamide were hydrolyzed by the PepR of L. rhamnosus.

Electrotransformation of Lactobacillus plantarum using linearized plasmid DNA

Evidence is presented that linearized plasmid DNA is capable of electrotransforming Lactobacillus plantarum at a frequency 500-fold lower than with the covalently closed circular molecule. When the linearized plasmid was religated prior to transformation the transformation efficiency was < 10-fold higher, suggesting that open circular plasmid was only slightly more efficient in the transformation of Lact. plantarum than linear DNA. This observation has implications for direct cloning into this species since the high background transformation frequency produced by the linear DNA could potentially obscure the recovery of clones. Nevertheless, using positive selection for enhanced chloramphenicol resistance, cloned fragments of Lact. helveticus DNA were obtained using the shuttle vector pGKV110.

Mutations to antibiotic resistance occur during the stationary phase in Lactobacillus plantarum ATCC 8014

When Lactobacillus plantarum ATCC 8014 was maintained in LCM broth (which consists of buffered tryptone and is sufficient to support the growth of some species of Lactobacillus ) for long periods (120 d), viable bacteria persisted. Rifampicin-, streptomycin- and sodium-fusidate-resistant mutants were recovered from parallel LCM broth cultures following a stochastic pattern. Individual cultures appeared to yield mutants intermittently. One culture in particular yielded rifampicin-resistant colonies at a frequency of 1 in 100 viable bacteria after 20 d incubation and these persisted until the experiment was terminated at 115 d. In a separate experiment two parallel cultures yielded mutants resistant to low concentrations of streptomycin at a similar frequency. Using a chemostat it was shown that in continuous culture in LCM at slow growth rates the highest frequency of recovery of antibiotic-resistant mutants was achieved when the bacteria exhibited doubling times of 90 h or greater. The frequency of recovery of mutants was as high as 1 in 1000 viable bacteria. Thus, mutations to antibiotic resistance in L. plantarum ATCC 8014 can take place in the absence of measurable cell division. The data are consistent with the notion that populations of starved bacteria in stationary phase can be genetically dynamic.

Technological and sensorial evaluation of Lactobacillus strains as starter cultures in fermented sausages

The performance of several lactobacilli strains isolated from naturally fermented sausages as starter cultures was evaluated. Microbiological, physical and chemical changes, in addition to sensorial aspects were studied. From the 12 different strains tested, 10 were capable of leading the fermentation in every batch throughout the process. The monitoring of the inoculated strains was easily carried out by plasmid profiles and by checking the pH rate drop of the sausages, which was slower for the non-inoculated lots. Treatment using natural fermentation resulted in a product where hydrogen sulphide odours, which could be related to the higher content of Enterobacteriaceae throughout the ripening process, diminished its overall acceptability. The lots seeded with different L. sake strains were found to be low in acid in sensory evaluation, correlating with a low lactic acid content. In contrast, the L. plantarum lot gave rise to an overacidified product related to having the highest amount of lactic acid at the end of the process. As a general rule lactobacilli strains isolated from meat origins are good candidates as starter cultures in the manufacture of dry-fermented sausages and produce satisfactory products depending on the specific strains used more than on the species.

Efficient transformation of Lactobacillus sake by electroporation

A procedure to transform intact Lactobacillus sake cells by electroporation was developed through a systematic examination of the effect of changes in various parameters on the transformation efficiency of Lact. sake strain 64F. The most critical factors were found to be the electrical parameters, the composition of washing and electroporation/storage solutions, and the presence of MgCI₂ in the expression medium. Under optimal conditions transformation efficiencies up to 10⁷ transformants (μg supercoiled DNA)^-1 were obtained. The optimized procedure was successfully applied to other Lact. sake strains and consistently yielded from 10⁴ to 10⁷ transformants (μg supercoiled DNA)^-1.