Autolysis of lactococci: detection of lytic enzymes by polyacrylamide gel electrophoresis and characterization in buffer systems

Survival of lactic acid and propionibacteria in low- and full-fat Dutch-type cheese during human digestion ex vivo

The survival of selected bacteria in semi-hard experimental cheeses was studied after exposure to human gastric and duodenal juices in an ex vivo model. Experimental cheeses (10 and 28% fat) were supplemented with different strains of Lactobacillus sp. and Propionibacterium sp. and ripened for 7 and 70 weeks. After digestion, greater numbers of the adjunct bacteria we rerecorded in the 7-week-old cheeses compared to the well-matured cheeses (70 weeks). The bacterial survival was strain dependent, and influenced by the fat content of the cheese. Lactobacilli showed better survival, especially when in low-fat cheeses. The strains of propionibacteria also survived well during the digestion of the low-fat cheeses. The results confirmed that cheese can potentially be a good carrier matrix for bacteria to the intestine. In addition, it has been shown that different strains present in cheese have different abilities to survive the conditions of the gastrointestinal tract. Younger cheese was indicated to be a better carrier, possibly because the bacteria present in those cheeses have had shorter exposure to the stress conditions occurring in cheese during prolonged maturation.

SIGNIFICANCE AND IMPACT OF THE STUDY

Cheese can function as a suitable vehicle for the delivery of a variety of food-related micro-organisms to the intestine. Young cheese as well as low-fat cheeses are better carrier matrixes than full-fat and/or more well-ripened cheeses. Most of the lactobacilli and all the propionibacteria survived well during digestion of the low-fat cheeses. This study also showed the ability of cheese lactobacilli and PAB to survive the severe conditions of GIT.

Construction and potential application of controlled autolytic systems for Lactobacillus casei in cheese manufacture

The rapid release of intracellular enzymes into the curd by the autolysis of lactic acid bacteria starters is universally recognized as a critical biological process to accelerate cheese ripening. Lactobacillus casei is typically the dominant nonstarter lactic acid bacterium in the ripening cheese. In this study, two controlled autolytic systems were established in L. casei BL23, based on the exploitation of the autolysins sourced from Lactococcus lactis (AcmA) and Enterococcus faecalis (AtlA). The lysis abilities of the systems were demonstrated both in broth and a model cheese, in which a fivefold increase in lactate dehydrogenase activity was detected in the curd with sufficient viable starter cells being maintained, indicating that they could lead to the timely release of intracellular enzymes.

Characterization of the Highly Autolytic Lactococcus lactis subsp. cremoris Strains CO and 2250

Two highly autolytic Lactococcus lactis subsp. cremoris strains (CO and 2250) were selected and analyzed for their autolytic properties. Both strains showed maximum lysis when grown in M17 broth containing a limiting concentration of glucose (0.4 to 0.5%) as the carbohydrate source. Lysis did not vary greatly with pH or temperature but was reduced when strains were grown on lactose or galactose. Growth in M17 containing excess glucose (1%) prevented autolysis, although rapid lysis of L. lactis subsp. cremoris CO did occur in the presence of 1% glucose if sodium fluoride (an inhibitor of glycolysis) was added to the medium. Maximum cell lysis in a buffer system was observed early in the stationary phase, and for CO, two pH optima were observed for log-phase and stationary-phase cells (6.5 and 8.5, respectively). Autolysins were extracted from the cell wall fraction of each strain by using either 4% sodium dodecyl sulfate (SDS), 6 M guanidine hydrochloride, or 4 M lithium chloride, and their activities were analyzed by renaturing SDS-polyacrylamide gel electrophoresis on gels containing Micrococcus luteus or L. lactis subsp. cremoris CO cells as the substrate. More than one lytic band was observed on each substrate, with the major band having an apparent molecular mass of 48 kDa for CO. Each lytic band was present throughout growth and lysis. These results suggest that at least two different autolytic enzymes are present in the autolytic L. lactis subsp. cremoris strains. The presence of the lactococcal cell wall hydrolase gene, acmA (G. Buist, J. Kok, K. J. Leenhouts, M. Dabrowska, G. Venema, and A. J. Haandrikman, J. Bacteriol. 177:1554-1563, 1995), in strains 2250 and CO was confirmed by Southern hybridization. Analysis of an acmA deletion mutant of 2250 confirmed that the gene was involved in cell separation and had a role in cell lysis.

Characterization of the bacteriolytic activity of nutritionally variant streptococci

The bacteriolytic activity of nutritionally variant streptococci (NVS), fastidious microaerophilic bacteria, which are members of the genera Abiotrophia and Granulicatella, was characterized in a renaturating SDS polyacrylamide gel electrophoresis system. Bacteriolytic profiles appeared quite different for the three species of NVS examined. Granulicatella adiacens or Abiotrophia defectiva each presented at least seven lytic bands, four of which were in common, while the other three were species-specific, whereas Granulicatella elegans showed six bands, which were overlapping with the G. adiacens bands. Four lytic bands were identified for enzymatic activity; D-alanyl-L-lysine hydrolase, endo-N-acetylglucosaminidase, endoacetylmuramidase, D-glutamyl-L-lysine hydrolase and acetylmuramoyl-L-alanine amidase activities could be defined. The bacteriolytic enzymes were purified and characterized for the kinetics of production during growth, autolytic activity, temperature and pH stability.

Effect of growth conditions of Lactobacillus gasseri OLL2809 on the immunostimulatory activity for production of interleukin-12 (p70) by murine splenocytes

Lactobacillus gasseri OLL2809, a probiotic lactic acid bacterium, strongly stimulates interleukin (IL)-12 (p70) production by murine splenocytes; therefore, it is expected to ameliorate allergic diseases. Although many studies have investigated characteristics of the immunostimulatory activity of probiotics, little is known about how bacterial growth conditions affect the activity. In this study, we investigated the effects of the growth conditions of L. gasseri OLL2809 on the stimulation of IL-12 (p70) production. L. gasseri OLL2809 was grown under various culture conditions including different cultivation periods, media, and culture pH, and IL-12 (p70) production by murine splenocytes stimulated with these bacterial cells was determined. The results revealed that IL-12 (p70) production (i) increased with the growth of the bacterial cells and was higher in stationary-phase cells than in logarithmic-phase cells; (ii) it was higher in the cells grown in acidic pH; and (iii) it decreased when the cells were incubated in a buffer at neutral pH prior to heat treatment. These observations indicated that stimulation of IL-12 (p70) production is affected by culture medium pH. In addition, the observations of a difference in the stimulation of IL-12 (p70) production by L. gasseri OLL2809 grown under various conditions are consistent with the characteristics of autolysis. Therefore, it was deduced that the integrity of the bacterial cells is necessary for the stimulatory effect on IL-12 (p70) production and that acidic pH and heat treatment contributed to the stimulation by inhibiting the activity of autolysins indigenous to the bacteria. Our result suggests that cultivation until the stationary phase under acidic pH is required for the effective production of probiotics with immunostimulatory activity.

Autolysis of propionibacteria: Detection of autolytic enzymes by renaturing SDS-PAGE and additional buffer studies

Five strains of propionibacteria with 70-90% autolysis in sodium lactate broth (SLB) were studied by renaturing sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). Several lytic bands ranging in size between 25 and 143 kDa were detected by using propionibacteria cells or cell walls as substrate in the gel. Four Propionibacterium freudenreichii strains showed similar autolytic-enzyme profiles, consisting of two autolytic bands, one with molecular mass 162 kDa and one in the range 123-143 kDa. However, the Propionibacterium acidipropionici strain showed a completely different profile, consisting of 8 autolytic bands with molecular masses of 122, 97, 71, 55, 43, 39, 31, and 25 kDa. Lytic enzymes from P. freudenreichii INF-alpha, P. freudenreichii ISU P-59, P. freudenreichii ISU P-24, and P. freudenreichii ISU P-50 showed lytic activity against cells from all these four strains, but not against P. acidipropionici ATCC 4965. However, P. acidipropionici ATCC 4965 autolysed only its own cells. Effects of pH, temperature, and ions on autolytic activity were tested by renaturing SDS-PAGE and in buffer systems. Results from the SDS-PAGE electrophoresis showed optimal autolytic activity of P. acidipropionici ATCC 4965 at 37 degrees C and in the pH range 7 to 8.5 and of P. freudenreichii ISU P-59 at 20 degrees C and in the pH range 5 to 7. The autolytic activity of P. acidipropionici ATCC 4965 was extremely heat stable (100 degrees C, 2 h), in contrast to the lytic activity of P. freudenreichii ISU P-59, which was heat labile. The autolytic activities of P. acidipropionici ATCC 4965 were inhibited by divalent cations, however, the lytic activities of P. freudenreichii ISU P-59 were activated by Mn(2+), Ca(2+), and Co(2+). In buffer, optimum autolysis of P. acidipropionici ATCC 4965 was observed at pH 8.5 and at 40 degrees C. P. freudenreichii ISU P-59 showed optimum autolysis in buffer at pH 7.5 and at 30 degrees C.

The autolytic phenotype of the Bacillus cereus group

AIM

To determine the autolytic phenotype of five species in the Bacillus cereus group.

METHODS AND RESULTS

The autolytic rate of 96 strains belonging to five species in the B. cereus group was examined under starvation conditions at pH 6, 6.5 and 8.5 in different buffers. The autolytic rate was strain-dependent with a wide variability at pH 6, but higher and more uniform at pH 6.5. At pH 8.5, and respect to the extent of autolysis at pH 6.5, it was relatively low for most of the strains with the lowest values between 13 and 52% in Bacillus mycoides and Bacillus pseudomycoides. Peptidoglycan hydrolase patterns evaluated by renaturing sodium dodecyl sulfate-polyacrylamide gel electrophoresis using cells of Bacillus thuringiensis ssp. tolworthi HD125 as an indicator, revealed complex profiles with lytic bands of about 90, 63, 46, 41, 38, 32, 28 and 25 kDa in B. cereus, B. thuringiensis and Bacillus weihenstephanensis. Bacillus mycoides and B. pseudomycoides had simpler profiles with lytic bands of 63, 46 and 38 kDa. Changes in the autolytic pattern were observed for cells harvested at the stationary phase of growth (72 h) showing an increase in the intensity of the 25 kDa band in the case of B. cereus, B. thuringiensis and B. weihenstephanensis, while no changes were observed for B. mycoides. Using Micrococcus lysodeicticus and Listeria monocytogenes as indicators lytic activity was retained by proteins of 63, 46, 38, 32 and 25 kDa and a new one of about 20 kDa in B. mycoides. Growth in the different media did not affect the autolytic pattern. NaCl abolished the activity of all the peptidoglycan hydrolases except for those of B. mycoides and B. weihenstephanensis. Lytic activity was retained in the presence of MgCl(2), MnCl(2) and EDTA and increased at basic pH.

CONCLUSIONS

Bacillus cereus/B. thuringiensis/B. weihenstephanensis showed a high extent of autolysis around neutral pH, even though they presented relatively complex autolysin profiles at alkaline pH. Bacillus mycoides/B. pseudomycoides had a higher extent of autolysis at acidic pH and a simpler autolysin pattern.

SIGNIFICANCE AND IMPACT OF THE STUDY

Information on the autolytic phenotype expand the phenotypic characterization of the different species in the B. cereus group.

Characterization of lytic enzyme activities of Lactobacillus gasseri with special reference to autolysis

Lactobacillus gasseri JCM 1130 and JCM 1131(T) exhibited autolytic activity in agar containing autoclaved cells of each strain as substrate. By zymogram analysis of JCM 1131(T), two lytic bands with apparent molecular masses of 54.5 and 35 kDa, were detected. Similarly, JCM 1130 yielded two lytic bands with apparent molecular masses of 35 and 33.5 kDa. In simple buffers as well, JCM 1131(T) suffered a drastic decrease in cell turbidity, but JCM 1130 did not undergo the decrease. The optimal pH for autolysis of JCM 1131(T) was in the range of 6.0-7.0, and the lysis was completely inhibited at pH 4-5. The lysis of JCM 1131(T) was suppressed by NaCl, in a concentration-dependent way. When subjected to UV irradiation or mitomycin C (MMC) treatment, cultures of both strains elicited conspicuous turbidity decrease after 2-4 h of growth, suggesting the occurrence of prophage induction. The 35-kDa lytic band of JCM 1131(T) and the 33.5-kDa protein of JCM 1130 were considerably increased by UV irradiation.

The autolytic phenotype of Bacillus thuringiensis

AIMS

To evaluate the autolytic phenotype of Bacillus thuringiensis.

METHODS AND RESULTS

The autolytic rate of 87 strains belonging to different subsp. of B. thuringiensis was examined at pH 6, 6.5 and 8.5 in different buffers under starvation conditions. At pH 6 the extent of autolysis (average in the strain collection 38.3 +/- 21.1) was strain-dependent with wide variability, while at pH 6.5 and 8.5 (averages 72.0 +/- 9.0 and 63.1 +/- 8.2, respectively) it was much more uniform with only a few strains showing low autolytic rates. Forty-one per cent of the strains showed high resistance (>/=80%) to mutanolysin, a commercial muramidase from Streptomyces. The peptidoglycan hydrolase pattern was evaluated by renaturing SDS-PAGE using cells of B. thuringiensis subsp. tolworthi HD125 as indicator. The strain collection showed seven major lytic bands of about 90, 63, 46, 38, 32, 28 and 25 kDa, and in the stationary growth phase (72 h) there was a more intense 25 kDa band in the autolytic pattern. Using Micrococcus lysodeicticus and Listeria monocytogenes as the indicators lytic activity was retained, as seen by the bands of 63, 46, 38, 32 and 25 kDa. Growth in the different media did not affect the autolytic pattern. NaCl abolished the activity of all the peptidoglycan hydrolases in the gel, but in the presence of KCl, MgCl(2), MnCl(2) and EDTA some activity was retained. At basic pH the lytic activity increased.

CONCLUSIONS

The autolytic phenotype of B. thuringiensis was found to be strain-dependent, and different proteins exibited peptidoglycan hydrolase activity, particularly at alkaline pH. Several of these proteins retained lytic activity against other bacterial species.

SIGNIFICANCE AND IMPACT OF THE STUDY

The characterisation of the autolytic phenotype of B. thuringiensis should expand the prospects of using this species in bacterial bio-control and field applications.

Phenotypic and PCR-based characterization of the microflora in Norvegia cheese during ripening

Microbiological sampling of Norvegia cheese from three cheese factories was done during ripening. The evolution of aerobic mesophilic bacteria, lactococci, lactobacilli, enterococci, presumptive leuconostoc and pediococci was investigated after 30, 90, 180 and 270 days of ripening. Isolates (135) of non-starter lactic acid bacteria (NSLAB) from nine Norvegia cheeses after 90, 180 and 270 days of ripening were examined. The isolates were tested by physiological and biochemical assays, species-specific PCR and 16S rDNA sequencing. After 90 days of ripening Leuconostoc spp., most probably from the starter, and the NSLAB specie Lactobacillus paracasei dominated among the isolates, however, after longer ripening Lb. paracasei dominated. The development and evolution of the microflora in Norvegia varied according to dairy and ripening time.

Effects of high pressure on the viability, morphology, lysis, and cell wall hydrolase activity of Lactococcus lactis subsp. cremoris

Viability, morphology, lysis, and cell wall hydrolase activity of Lactococcus lactis subsp. cremoris MG1363 and SK11 were determined after exposure to pressure. Both strains were completely inactivated at pressures of 400 to 800 MPa but unaffected at 100 and 200 MPa. At 300 MPa, the MG1363 and SK11 populations decreased by 7.3 and 2.5 log cycles, respectively. Transmission electron microscopy indicated that pressure caused intracellular and cell envelope damage. Pressure-treated MG1363 cell suspensions lysed more rapidly over time than did non-pressure-treated controls. Twenty-four hours after pressure treatment, the percent lysis ranged from 13.0 (0.1 MPa) to 43.3 (300 MPa). Analysis of the MG1363 supernatants by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) confirmed pressure-induced lysis. Pressure did not induce lysis or membrane permeability of SK11. Renaturing SDS-PAGE (zymogram analysis) revealed two hydrolytic bands from MG1363 cell extracts treated at all pressures (0.1 to 800 MPa). Measuring the reducing sugars released during enzymatic cell wall breakdown provided a quantitative, nondenaturing assay of cell wall hydrolase activity. Cells treated at 100 MPa released significantly more reducing sugar than other samples, including the non-pressure-treated control, indicating that pressure can activate cell wall hydrolase activity or increase cell wall accessibility to the enzyme. The cell suspensions treated at 200 and 300 MPa did not differ significantly from the control, whereas cells treated at pressures greater than 400 MPa displayed reduced cell wall hydrolase activity. These data suggest that high pressure can cause inactivation, physical damage, and lysis in L. lactis. Pressure-induced lysis is strain dependent and not solely dependent upon cell wall hydrolase activity.

Identification of Mur, an atypical peptidoglycan hydrolase derived from Leuconostoc citreum

A gene encoding a protein homologous to known bacterial N-acetyl-muramidases has been cloned from Leuconostoc citreum by a PCR-based approach. The encoded protein, Mur, consists of 209 amino acid residues with a calculated molecular mass of 23,821 Da including a 31-amino-acid putative signal peptide. In contrast to most of the other known peptidoglycan hydrolases, L. citreum Mur protein does not contain amino acid repeats involved in cell wall binding. The purified L. citreum Mur protein was shown to exhibit peptidoglycan-hydrolyzing activity by renaturing sodium dodecyl sulfate-polyacrylamide gel electrophoresis. An active chimeric protein was constructed by fusion of L. citreum Mur to the C-terminal repeat-containing domain (cA) of AcmA, the major autolysin of Lactococcus lactis. Expression of the Mur-cA fusion protein was able to complement an acmA mutation in L. lactis; normal cell separation after cell division was restored by Mur-cA expression.

Autolysis of dairy leuconostocs and detection of peptidoglycan hydrolases by renaturing SDS-PAGE

The autolysis of lactic acid bacteria plays a major role during cheese ripening. The aim of this study was to evaluate the autolytic properties and peptidoglycan hydrolase content of dairy leuconostocs. Autolysis of 59 strains of dairy Leuconostoc was examined under starvation conditions in potassium phosphate buffer. The ability of dairy leuconostocs to lyse is strain dependant and not related to the species. The peptidoglycan hydrolase profile of Leuc. mesenteroides subsp. mesenteroides 10L was analysed by renaturing gel electrophoresis. Two major activity bands migrating at 41 and 52 kDa were observed. According to the specificity analysis, strain 10L seems to contain a glycosidase and an N-acetyl-muramyl-L-alanine amidase, or an endopeptidase. The peptidoglycan hydrolase profiles of various Leuconostoc species were also compared. Several peptidoglycan hydrolase activities could be detected in the different Leuconostoc species. Further characterization of the peptidoglycan hydrolases will help to control autolysis of leuconostocs in cheese.

Requirement of autolytic activity for bacteriocin-induced lysis

The bacteriocin produced by Lactococcus lactis IFPL105 is bactericidal against several Lactococcus and Lactobacillus strains. Addition of the bacteriocin to exponential-growth-phase cells resulted in all cases in bacteriolysis. The bacteriolytic response of the strains was not related to differences in sensitivity to the bacteriocin and was strongly reduced in the presence of autolysin inhibitors (Co(2+) and sodium dodecyl sulfate). When L. lactis MG1363 and its derivative deficient in the production of the major autolysin AcmA (MG1363acmADelta1) were incubated with the bacteriocin, the latter did not lyse and no intracellular proteins were released into the medium. Incubation of cell wall fragments of L. lactis MG1363, or of L. lactis MG1363acmADelta1 to which extracellular AcmA was added, in the presence or absence of the bacteriocin had no effect on the speed of cell wall degradation. This result indicates that the bacteriocin does not degrade cell walls, nor does it directly activate the autolysin AcmA. The autolysin was also responsible for the observed lysis of L. lactis MG1363 cells during incubation with nisin or the mixture of lactococcins A, B, and M. The results presented here show that lysis of L. lactis after addition of the bacteriocins is caused by the resulting cell damage, which promotes uncontrolled degradation of the cell walls by AcmA.

Investigation of the relationship between lysogeny and lysis of Lactococcus lactis in cheese using prophage-targeted PCR

The ability of lactococcal strains to lyse (and release intracellular enzymes) during cheese manufacture can be a very desirable trait and has been associated with improvement in flavor and acceleration of cheese ripening. Using a laboratory-scale cheese manufacturing assay, the autolytic behavior of 31 strains of Lactococcus lactis was assessed. In general, marked variation was observed between strains with a 20-fold difference between the best and worst lysing strains based on the release of the intracellular enzyme lactate dehydrogenase. In a parallel experiment, the genomes of these strains were examined for the presence of prophage integrase (int) sequences by using conserved primer sequences from known lysogenic phage. Results demonstrated that the lytic behavior of lactococcal starter strains significantly correlates with the presence of prophage sequences. These results highlight not only the contribution of prophage to starter cell lysis but also the potential of PCR as a useful initial screen to assess strains for this important industrial trait.

The Streptococcus thermophilus autolytic phenotype results from a leaky prophage

Streptococcus thermophilus autolytic strains are characterized by a typical bell-shaped growth curve when grown under appropriate conditions. The cellular mechanisms involved in the triggering of lysis and the bacteriolytic activities of these strains were investigated in this study. Lactose depletion and organic solvents (ethanol, methanol, and chloroform) were shown to trigger a premature and immediate lysis of M17 exponentially growing cells. These factors and compounds are suspected to act by altering the cell envelope properties, causing either the permeabilization (organic solvents) or the depolarization (lactose depletion) of the cytoplasmic membrane. The autolytic character was shown to be associated with lysogeny. Phage particles, most of which were defective, were observed in the culture supernatants after both mitomycin C-induced and spontaneous lysis. By renaturing sodium dodecyl sulfate-polyacrylamide gel electrophoresis, a bacteriolytic activity was detected at 31 kDa exclusively in the autolytic strains. This enzyme was detected during both growth and spontaneous lysis with the same intensity. We have shown that it was prophage encoded and homologous to the endolysin Lyt51 of the streptococcal temperate bacteriophage phi01205 (M. Sheehan, E. Stanley, G. F. Fitzgerald, and D. van Sinderen, Appl. Environ. Microbiol. 65:569-577, 1999). It appears from our results that the autolytic properties are conferred to the S. thermophilus strains by a leaky prophage but do not result from massive prophage induction. More specifically, we propose that phagic genes are constitutively expressed in almost all the cells at a low and nonlethal level and that lysis is controlled and achieved by the prophage-encoded lysis proteins.

Analysis of the bacteriolytic enzymes of the autolytic lactococcus lactis subsp. cremoris strain AM2 by renaturing polyacrylamide gel electrophoresis: identification of a prophage-encoded enzyme

Lactococcus lactis subsp. cremoris AM2 was previously shown to lyse early and extensively during cheese ripening (M.-P. Chapot-Chartier, C. Deniel, M. Rousseau, L. Vassal, and J.-C. Gripon, Int. Dairy J. 4:251-269, 1994). We analyzed the bacteriolytic activities of autolytic strain AM2 by using renaturing sodium dodecyl sulfate-polyacrylamide gel electrophoresis performed with two different substrates in the gel, Micrococcus lysodeikticus and L. lactis autoclaved cells. Several lytic activities were detected in L. lactis AM2; a major lytic activity, designated A2 (46 kDa), was found only with the L. lactis cell substrate. This activity appears to be different from major peptidoglycan hydrolase AcmA characterized previously (G. Buist, J. Kok, K. J. Leenhouts, M. Dabrowska, G. Venema, and A. J. Haandrickman, J. Bacteriol. 177:1554-1563, 1995), which has a similar molecular mass. The two enzymes differ in substrate specificity as well as in sensitivity to pH and different chemical compounds. L. lactis AM2 is lysogenic and mitomycin C inducible. Enzyme A2 was shown to be inducible by mitomycin C and to be prophage encoded. It was identified as an enzyme similar to the lysin encoded by lactococcal small isometric temperate bacteriophages. A prophage-cured derivative of L. lactis AM2 was obtained, and this isolate exhibited different autolytic properties than AM2. After prolonged incubation in the stationary phase after growth on M17 medium, the extent of lysis of an AM2 culture was 60%, whereas over the same period there was almost no lysis in a prophage-cured derivative strain culture. These results suggest that the prophage lytic system is involved in the strain AM2 lysis observed in liquid medium and that it could also be involved in the lysis observed during cheese ripening.

A chloride-inducible gene expression cassette and its use in induced lysis of Lactococcus lactis

A chloride-inducible promoter previously isolated from the chromosome of Lactococcus lactis (J. W. Sanders, G. Venema, J. Kok, and K. Leenhouts, Mol. Gen. Genet., in press) was exploited for the inducible expression of homologous and heterologous genes. An expression cassette consisting of the positive-regulator gene gadR, the chloride-inducible promoter Pgad, and the translation initiation signals of gadC was amplified by PCR. The cassette was cloned upstream of Escherichia coli lacZ, the holin-lysin cassette (lytPR) of the lactococcal bacteriophage r1t, and the autolysin gene of L. lactis, acmA. Basal activity of Pgad resulted in a low level of expression of all three proteins. Growth in the presence of 0.5 M NaCl of a strain containing the gadC::lacZ fusion resulted in a 1,500-fold increase of beta-galactosidase activity. The background activity levels of LytPR and AcmA had no deleterious effects on cell growth, but induction of lysin expression by addition of 0.5 M NaCl resulted in inhibition of growth. Lysis was monitored by following the release of the cytoplasmic marker enzyme PepX. Released PepX activity was maximal at 1 day after induction of lytPR expression with 0.1 M NaCl. Induction of acmA expression resulted in slower release of PepX from the cells. The presence of the inducing agent NaCl resulted in the stabilization of osmotically fragile cells.

Autolysis of Lactococcus lactis caused by induced overproduction of its major autolysin, AcmA

The optical density of a culture of lactococcus lactis MG1363 was reduced more than 60% during prolonged stationary phase. Reduction in optical density (autolysis) was almost absent in a culture of an isogenic mutant containing a deletion in the major autolysin gene, acmA. An acmA mutant carrying multiple coples of a plasmid encoding AcmA lysed to a greater extent than the wild-type strain did. Intercellular action of AcmA was shown by mixing end-exponential-phase cultures of an acmA deletion mutant and a tripeptidase (pepT) deletion mutant. PepT, produced by the acmA mutant, was detected in the supernatant of the mixed culture, but no PepT was present in the culture supernatant of the acmA mutant. A plasmid was constructed in which acmA, lacking its own promoter, was placed downstream of the inducible promoter/operator region of the temperate lactococcal bacteriophage r1t. After mitomycin induction of an exponential-phase culture of L. lactis LL302 carrying this plasmid, the cells became subject to autolysis, resulting in the release of intracellular proteins.

Lytic systems in lactic acid bacteria and their bacteriophages

Lytic systems of lactic acid bacteria and their bacteriophages are reviewed with an emphasis on molecular characterization. Details of enzyme biochemistry and the cloning and analysis of lytic genes are presented, with coverage of lactococcal prolate headed bacteriophages, lactococcal isometric bacteriophages, Lactobacillus bacteriophages and lactococcal autolysins. Some comments on the importance of autolysis in cheese ripening are included and the biotechnological exploitation of cloned and characterized lytic genes is presented.