AbiQ, an abortive infection mechanism from Lactococcus lactis

Lactococcus lactis W-37 is highly resistant to phage infection. The cryptic plasmids from this strain were coelectroporated, along with the shuttle vector pSA3, into the plasmid-free host L. lactis LM0230. In addition to pSA3, erythromycin- and phage-resistant isolates carried pSRQ900, an 11-kb plasmid from L. lactis W-37. This plasmid made the host bacteria highly resistant (efficiency of plaquing <10(-8)) to c2- and 936-like phages. pSRQ900 did not confer any resistance to phages of the P335 species. Adsorption, cell survival, and endonucleolytic activity assays showed that pSRQ900 encodes an abortive infection mechanism. The phage resistance mechanism is limited to a 2.2-kb EcoRV/BclI fragment. Sequence analysis of this fragment revealed a complete open reading frame (abiQ), which encodes a putative protein of 183 amino acids. A frameshift mutation within abiQ completely abolished the resistant phenotype. The predicted peptide has a high content of positively charged residues (pI = 10.5) and is, in all likelihood, a cytosolic protein. AbiQ has no homology to known or deduced proteins in the databases. DNA replication assays showed that phage c21 (c2-like) and phage p2 (936-like) can still replicate in cells harboring AbiQ. However, phage DNA accumulated in its concatenated form in the infected AbiQ+ cells, whereas the AbiQ- cells contained processed (mature) phage DNA in addition to the concatenated form. The production of the major capsid protein of phage c21 was not hindered in the cells harboring AbiQ.

Phenotypic and genetic characterization of the bacteriophage abortive infection mechanism AbiK from Lactococcus lactis

The natural plasmid pSRQ800 isolated from Lactococcus lactis subsp. lactis W1 conferred strong phage resistance against small isometric phages of the 936 and P335 species when introduced into phage-sensitive L. lactis strains. It had very limited effect on prolate phages of the c2 species. The phage resistance mechanism encoded on pSRQ800 is a temperature-sensitive abortive infection system (Abi). Plasmid pSRQ800 was mapped, and the Abi genetic determinant was localized on a 4.5-kb EcoRI fragment. Cloning and sequencing of the 4.5-kb fragment allowed the identification of two large open reading frames. Deletion mutants showed that only orf1 was needed to produce the Abi phenotype. orf1 (renamed abiK) coded for a predicted protein of 599 amino acids (AbiK) with an estimated molecular size of 71.4 kDa and a pI of 7.98. DNA and protein sequence alignment programs found no significant homology with databases. However, a database query based on amino acid composition suggested that AbiK might be in the same protein family as AbiA. No phage DNA replication nor phage structural protein production was detected in infected AbiK+ L. lactis cells. This system is believed to act at or prior to phage DNA replication. WHen cloned into a high-copy vector, AbiK efficiency increased 100-fold. AbiK provides another powerful tool that can be useful in controlling phages during lactococcal fermentations.

Cloning and Sequencing of LlaII Restriction/Modification Genes from Lactococcus lactis and Relatedness of This System to the Streptococcus pneumoniae DpnII System

Volume 61, no. 6, p. 2193-2202: the restriction/modification system designation "LlaII" should read "LlaDCHI" throughout the paper to conform to conventional nomenclature for such systems. [This corrects the article on p. 2193 in vol. 61.].

Expression of a Lactococcus lactis Phage Resistance Mechanism by Streptococcus thermophilus

The 7.8-kb lactococcal plasmid pSRQ700 encodes the LlaII restriction/modification system which recognizes and cleaves the sequence 3(prm1)-GATC-5(prm1). When the plasmid pSRQ700 is introduced into a phage-sensitive Lactococcus lactis strain, strong phage resistance is conferred by the LlaII system. In this report, we show that pSRQ700 cannot replicate in Streptococcus thermophilus. However, if cloned into the vector pNZ123, the native LlaII system is expressed and strong phage resistance is conferred to various industrial S. thermophilus strains. Resistance against phages isolated from yogurt and mozzarella wheys was observed. To our knowledge, this is the first report of increased phage resistance in S. thermophilus.

Cloning and sequencing of LlaDCHI [corrected] restriction/modification genes from Lactococcus lactis and relatedness of this system to the Streptococcus pneumoniae DpnII system

The natural 7.8-kb plasmid pSRQ700 was isolated from Lactococcus lactis subsp. cremoris DCH-4. It encodes a restriction/modification system named LlaDCHI [corrected]. When introduced into a phage-sensitive L. lactis strain, pSRQ700 confers strong phage resistance against the three most common lactococcal phage species, namely, 936, c2, and P335. The LlaDCHI [corrected] endonuclease was purified and found to cleave the palindromic sequence 5'-GATC-3'. It is an isoschizomer of Streptococcus pneumoniae DpnII. The plasmid pSRQ700 was mapped, and the genetic organization of LlaDCHI [corrected] was localized. Cloning and sequencing of the entire LlaDCHI [corrected] system allowed the identification of three open reading frames. The three genes (llaIIA, llaIIB, and llaIIC) overlapped and are under one putative promoter. A putative terminator was found at the end of llaIIC. The genes llaIIA and llaIIB coded for m6A methyltransferases, and llaIIC coded for an endonuclease. The LlaDCHI [corrected] system shares strong genetic similarities with the DpnII system. The deduced amino acid sequence of M.LlaIIA was 75% identical with that of M.DpnII, whereas M.LlaIIB was 88% identical with M.DpnA. However, R.LlalII shared only 31% identity with R.DpnII.

Inhibition of Listeria monocytogenes by using bacteriocin PA-1 produced by Pediococcus acidilactici PAC 1.0

The bacteriocin produced by Pediococcus acidilactici PAC 1.0, previously designated PA-1 bacteriocin, was found to be inhibitory and bactericidal for Listeria monocytogenes. A dried powder prepared from PAC 1.0 culture supernatant fortified with 10% milk powder was found to contain bacteriocin activity. An MIC against L. monocytogenes and lytic effects in broth cultures were determined. Inhibition by PA-1 powder occurred over the pH range 5.5 to 7.0 and at both 4 and 32 degrees C. In addition, inhibition of L. monocytogenes was demonstrated in several food systems including dressed cottage cheese, half-and-half cream, and cheese sauce.

Phage resistance in Streptococcus lactis ssp. diacetylactis transconjugant SLA3.2501 and its derivatives

Phage 18-16, which was virulent for Streptococcus lactis ssp. diacetylactis SLA3.25 was used to study phage-resistant characteristics of mucoid S. lactis ssp. diacetylactis transconjugant SLA3.2501 obtained through conjugative cotransfer of pSRQ2201 (Lac-plasmid) and pSRQ2202 (Muc-plasmid) to SLA3.25 (15). Interaction of phage 18-16 with SLA3.2501 and its derivatives showed that phage resistance was not related to either the lack of phage adsorption or restriction-modification. Suppression of phage replication in SLA3.2501 and its derivatives was not completely relieved by curing of either pSRQ2201 or pSRQ2202 or both.

Involvement of a Plasmid in Production of Ropiness (Mucoidness) in Milk Cultures by Streptococcus cremoris MS

Curing and genetic transfer experiments showed that lactose-fermenting ability (Lac + ) and the ability to produce mucoidness in milk cultures (Muc + ) in Streptococcus cremoris MS were coded on plasmids. The Lac + phenotype was associated with a 75.8-megadalton plasmid, pSRQ2201. The Muc + phenotype was associated with a 18.5-megadalton plasmid, pSRQ2202. The Lac plasmid, pSRQ2201, was first conjugatively transferred from S. cremoris MS to Lac − S. lactis ML-3/2.2. Later, the Muc plasmid, pSRQ2202, was conjugatively transferred from Lac − Muc + S. cremoris MS04 to Lac + nonmucoid S. lactis transconjugant ML-3/2.201. Subsequently, pSRQ2201 and pSRQ2202 were cotransferred from Lac + Muc + S. lactis transconjugant ML-3/2.202 to Lac − , nonmucoid, malty S. lactis 4/4.2 and S. lactis subsp. diacetylactis SLA3.25. Transconjugants showing pSRQ2201 were Lac + ; those containing pSRQ2202 were Muc + . With the transfer of pSRQ2202, the transconjugants S. lactis ML-3/2.202 and S. lactis subsp. diacetylactis SLA3.2501 not only acquired the Muc + phenotype but also resistance to bacteriophages, which were lytic to the respective parent strains S. lactis ML-3/2.201 and S. lactis subsp. diacetylactis SLA3.25.

Heat-Resistant Psychrotrophic Bacteria Isolated from Pasteurized Milk 1

Psychrotrophic bacteria were isolated from 227 pasteurized milk samples which had a shelf life in excess of 20 days at 7.2 C. Of 700 cultures isolated, 135 were resistant to heating at 72 C for 16 sec and were able to re-establish growth at 7.2 C. Thirty-five cultures, representing 15 different types were subjected to detailed examination to determine their actions on refrigerated milk, growth temperatures, thermal resistance at various temperatures, and their identities. The spore-forming genus Bacillus occured most frequently. The non-sporing types were assigned to the genera Arthrobacter , Microbacterium , Streptococcus , and Corynebacterium .

Beta-galactosidase of Propionibacterium shermanii

Ten strains of Propionibacterium shermanii were tested for beta-galactosidase (beta-gal) activity. Of these ten strains, five yielded enhanced enzyme activity when cell suspensions were treated with toluene-acetone; on solvent treatment, the remaining five lost a considerable portion of the activity found in whole-cell suspensions. By using a strain yielding decreased activity upon solvent treatment, explanations for the loss in activity were sought through assays for possible alternative beta-galactoside utilization mechanisms. When this strain was assayed for beta-D-phosphogalactoside galactohydrolase by using orthonitrophenyl-beta-D-galactopyranoside-6-P04 as a substrate, the activity was wither lower or indiffernt as compared with beta-gal activity determined simultaneously. Cell suspensions of P. shermanii 7 and 22 (strains chosen for further work) grown separately on the individual substrates (lactose, glucose, galactose, and sodium lactate) did not show significant differences in beta-gal activity. Optimal temperature for beta-gal activity in untreated and toluene-acetone-treated cell suspensions of strain 7 was 52 C. With strain 22, of the temperatures tested, maximal activity in untreated cell suspensions was noted at 58 C and with solvent-treated cells at 32 C. In the cell-free extract (CFE) system, both strains exhibited maximal activity at 52 C. Optimal pH for untreated and solvent-treated cell suspensions of both strains was around 7.5. In the P. shermanii 22 CFE system, maximal activity occurred at pH 7.0; pH had very little effect on enzyme activity in P. shermanii 7 CFE. Sodium or potassium phosphate buffers in the assay system yielded the best activity. In the CFE system of these two strains, Mn2+ was definitely stimulatory, but in untreated and solvent-treated cell systems of these strains presence or absence of Mn2+ in the assay system had variable effects on enzyme activity. Maximal beta-gal activity was noted in P. shermanii 7 cells harvested after 28 h of growth at 32 C in sodium lactate broth. Sulfhydryl-group blocking agents inhibited enzyme activity in P. shermanii 22 CFE; the inhibition was partly reversed by dithiothreitol.

CAPSULATION OF PROPIONIBACTERIUM1

Strains of nine species of Propionibacterium were examined for slime and/or capsule formation. Direct evidence of capsules was demonstrated by negative staining with India ink. Changes in viscosity of growth media, estimated using an Ostwald viscosimeter, were related to capsule or slime production. Mucoid colony production provided further evidence of slime formation. Under the criteria and experimental conditions adopted, at least one strain of each species and 34 of 82 strains formed slime. Incubation temperatures of 15 and 21 C which are less than optimal for growth, adjustment of media pH above neutrality, addition of sodium chloride, and as many as 11 different carbohydrates were conducive to slime formation. Paper and thin-layer chromatographic analyses of acid-hydrolyzed capsular material indicated that it contained mannose and lesser amounts of glucose and galactose.

SPLIT DEFECT OF SWISS CHEESE

When strains of propionibacteria able to grow at 3.8 C were used in Swiss cheese manufacture, resulting 3- and 6-month-old cheeses had a high incidence of splits (6 of 13 and 10 of 13 lots, respectively). With strains lacking this low-temperature growth ability, only 1 of 10 and 2 of 10 lots of cheese split after comparable curing periods. Degree of cheese proteolysis, moisture, and sugar content could not be related to Propionibacterium strain used nor to split incidence. Carbon dioxide production in the warm (21 C) room by strains able to grow at low temperatures was essentially twice that of strains lacking this ability. This CO2 production difference between strains was maintained during subsequent cold-room curing. Type of cheese-wrapping film also affected split incidence. Commercially manufactured cheeses wrapped in four films of differing gas permeability showed a distinct tendency toward splitting in the films impermeable to oxygen.

CHANGES IN MILK, WHEY, AND BLUE CHEESE AS INDUCED BY BENZOYL PEROXIDE1,2

Milk proteins were subjected to treatment with various levels of benzoyl peroxide, with and without heating at 60 C for 2 h. Heating had a pronounced effect on whey proteins, but polyacrylamide gel electrophoresis revealed changes in proteins not attributable to heat alone. The effect on proteins was reflected in an increased tendency for the benzoyl peroxide-heat treated cheeses to expel moisture during leakage tests. Use of 17.8 ppm benzoyl peroxide resulted in a markedly whiter cheese than that made using 5.9 ppm and reflectance studies indicated this to be true even when no heat treatment accompanied the benzoyl peroxide. Use of benzoyl peroxide in the bleaching process did not decrease mold development in ripening loaves nor was acid production by lactic cultures diminished. In addition, proteolysis of milk proteins by rennet was not reduced by the presence of benzoyl peroxide.

AN AGAR MEDIUM FOR THE DIFFERENTIAL ENUMERATION OF YOGURT STARTER BACTERIA1,2

Generally, Streptococcus thermophilus ferments sucrose, whereas Lactobacillus bulgaricus fails to utilize this disaccharide. When both sucrose and lactose were added to a basal medium, S. thermophilus fermented both carbon sources, produced sufficient acid to change the color of an acid-base indicator (bromcresol purple), and hence formed yellow colonies. On the same medium, most L. bulgaricus strains grew more slowly, produced less acid, and yielded white colonies. Acid diffusion around the S. thermophilus colonies was localized by incorporation of CaCO3 into the medium. To test the efficacy of this medium when known strains of starters are used, the effect of freezing with liquid nitrogen on mixed cultures of S. thermophilus and L. bulgaricus was studied.

CHARACTERISTICS OF CHEDDAR CHEESE COOLED AT DIFFERENT RATES DURING EARLY CURING STAGES

Sixteen lots of Cheddar cheese were made from manufacturing-grade and grade-A milk by using commercial starters and cultures reported to give fruity and bitter-flavored cheese. Curd blocks were pressed for 4 and 20 h, then cooled to 7.5 C in brine or in air. There were no statistically significant differences in total, enterococcus, and Violet red bile agar counts, or proteolysis and judging scores among different pressing or cooling treatments. Significantly greater amounts of lactose, glucose, and galactose were present in brine-cooled cheese. Lactose persisted beyond 3 months. Air-cooled cheese had a significantly higher lactic-acid content, more free fatty acids, and more color variation. Air-cooled cheese was more severely criticized for flavor defects. Brine cooling produced uniform flavor, body, and color, and six of the seven judges preferred brine-cooled cheese.

ROLE OF ENTEROCOCCI IN CHEDDAR CHEESE: GROWTH OF ENTEROCOCCI DURING MANUFACTURE AND CURING1

Eight lots of Cheddar cheese were manufactured to determine the microbiological response of two strains each of Streptococcus faecalis and Streptococcus durans when used as supplemental starters in combination with a commercial lactic culture. Each lot consisted of a control vat of cheese manufactured with the lactic starter only, and an experimental vat of cheese containing the lactic starter and one of the enterococcus strains. Combinations of two curing temperatures ( 7.2 and 12.8 C) and two early cooling treatments (air vs. brine cooling) were used for cheeses from each vat to determine environmentally-induced variability.

Growth patterns were monitored throughout the manufacture period up to the end of pressing, and during curing up to 6 months. Enterococcus populations showed little or no decrease when the cheeses were being pressed, whereas populations in control cheeses decreased over the same period. During curing, control cheeses cured at 7.2 C showed marked population decreases over the 6 months; those cured at 12.8 C showed a rapid decrease followed by an upsurge in population. Populations of S. faecalis in the experimental cheeses decreased only slightly, and S. durans showed almost no decrease. Generally, cheeses cured at 7.2 C showed the greatest numerical survival and there appeared to be no population differences caused by early cooling treatment.

IMMUNOGENIC PROPERTIES OF SLIME FROM PROPIONIBACTERIA1

Slime, produced by members of the genus Propionbacterium, was isolated, and attempts were made with it to induce antibody formation in biological systems to demonstrate the possible use of anticapsular antibodies for serological typing. Slime produced by three different species of Propionbacterium did not produce detectable precipitating antibodies in rabbits. Killed whole cells of Propionbacterium Zeae induced specific agglutinin formation. Antibodies produced against whole cell antigens of P. freudenreichii and P. shermanii exhibited substantial heterologous agglutination reactions.

Agar Medium for Differential Enumeration of Lactic Streptococci

An agar medium containing arginine and calcium citrate as specific substrates, diffusible (K 2 HPO 4 ) and undiffusible (CaCO 3 ) buffer systems, and bromocresol purple as the p H indicator was developed to differentiate among lactic streptococci in pure and mixed cultures. Milk was added as the sole source of carbohydrate (lactose) and to provide growth-stimulating factors. Production of acid from lactose caused developing bacterial colonies to seem yellow. Subsequent arginine utilization by Streptococcus lactis and S. diacetilactis liberated ammonia, resulting in a localized p H shift back toward neutrality and a return of the original purple indicator hue. The effects of production of acid from lactose and ammonia were fixed around individual colonies by the buffering capacity of CaCO 3 . After 36 hr at 32 C in a candle oats jar, colonies of S. cremoris were yellow, whereas colonies of S. lactis and S. diacetilactis were white. S. diacetilactis , on further incubation, utilized suspended calcium citrate, and, after 6 days, the citrate-degrading colonies exhibited clear zoning against a turbid background, making them easily distinguishable from the colonies of the other two species. The medium proved suitable for quantitative differential enumeration when compared with another widely used general agar medium for lactic streptococci.

Associative Growth Studies in Three-Strain Mixtures of Lactic Streptococci

A recently developed differential agar medium was used to study associative growth patterns in 17 different heterologous, three-strain mixtures of Streptococcus lactis, S. cremoris , and S. diacetilactis grown in milk. Mixtures were made by combining equal volumes of 18-hr milk cultures of the three species. Relative populations of component species were followed through three successive transfers in milk after the initial mixed propagation. Direct evidence for strain dominance and compatibility was obtained. A procedure also was developed to estimate the extent of suppression of S. lactis and S. diacetilactis in a mixture containing a dominant S. cremoris strain. The technique described could be successfully applied in quality-control work in the dairy-starter manufacturing industry.

Isolation of inhibitory factor in raw milk whey active against propionibacteria

Preparative isolation of the active component(s) in skim milk whey inhibitory for propionibacteria was made by using (NH(4))(2)SO(4) salt fractionation. The crude preparation was further purified by Sephadex G-100 column separation. Disc-gel electrophoresis of the active peak from the Sephadex elution pattern (peak I) showed that this fraction contained almost all of the immune globulin in the column sample. The biologically inactive peaks did not contain any immune globulin. Starch-gel electrophoresis of the active peak revealed the presence of three separate immune globulin fractions. A correlation was also observed between hemolytic reaction of propionibacterial strains and relative resistance to whey inhibition. The investigation showed that one of the immune globulins of milk, pseudoglobulin, was mainly responsible for the suppressive activity of whey.

ASSOCIATIVE GROWTH RELATIONSHIPS IN TWO STRAIN MIXTURES OF STREPTOCOCCUS LACTIS AND STREPTOCOCCUS CREMORIS1

A recently described differential agar medium was used to study strain interactions in two-strain mixtures of Streptococcus lactis and Streptococcus cremoris. Two S. cremoris strains (ML4 and DR7) exhibited marked dominance over four S. lactis cultures. One S. cremoris strain, designated 1, showed excellent compatibility in all combinations. Streptococcus cremoris HP was progressively suppressed by all S. lactis strains. The associative growth patterns at 32 C and 21 C were similar irrespective of the initial cell numbers of the component strains.

The technique described in this paper could be used in conjunction with phage tracer methods to investigate growth relationships among mixed strain lactic starters containing more than one strain each of S. lactis and S. cremoris.

A DIFFERENTIAL BROTH FOR SEPARATING THE LACTIC STREPTOCOCCI1

Arginine degradation and citrate utilization, the major differentiating characteristics among lactic streptococci, formed the basis of a differential broth for separating Streptococcus cremoris, Streptococcus lactis, and Streptococcus diacetilactis strains in pure cultures.

The medium contains milk as the sole source of carbohydrate (lactose), arginine and sodium citrate as specific substrates, and a suitable pH indicator (bromcresol purple), in addition to other ingredients. The pH of the medium is adjusted to 6.2 ± 0.05 (which becomes 6.15 ± 0.05 after sterilization) to increase citrate utilization and the broth is dispensed into test tubes containing Durham fermentation tubes. Streptococcus cremoris produces a yellow reaction (acid) in the broth. Streptococcus lactis initially turns the broth yellow, but on liberation of NH3 reverses the color back to the original violet hue. Streptococcus diacetilactis produces a violet reaction, and CO2 accumulates in the Durham fermentation tubes from the fermentation of sodium citrate.

Diacetyl production by Propionibacterium shermanii in milk cultures

Diacetyl production by propionibacteria in milk cultures was studied using a previously described sensitive method specific for diacetyl. Twenty-four strains belonging to six Propionibacterium species were included in the study. A strain of P. shermanii that produced relatively large amounts of diacetyl in milk was chosen for investigations on factors affecting diacetyl production.Accumulation of the diketone in milk cultures was greater at 21 C than at 32 or 37 C although the rate of production of the compound was greater at the higher temperatures. Rapid cooling after incubation stabilized the diacetyl levels in milk cultures, and prolonged refrigerated holding caused an increase in the concentration of the flavor compound. The dicarbonyl production was greatly affected by the pH of the culture medium, the most favorable range being 4.0 to 4.5. In mixed culture study using a homo-fermentative Streptococcus lactis in combination with the P. shermanii strain, diacetyl production by the latter was greatly enhanced by the rapid pH depression brought about by the streptococci. Diacetyl reductase assays on cell-free extract preparations from the P. shermanii strain showed little or no activity.

LIPOLYTIC AND PROTEOLYTIC ACTIVITY OF ENTEROCOCCI AND LACTIC GROUP STREPTOCOCCI ISOLATED FROM YOUNG CHEDDAR CHEESE1

Lipolytic and proteolytic screening techniques were applied to cultures isolated from young Cheddar cheese manufactured in 10 Iowa cheese plants. Twenty-one cultures were selected for study. These included 16 enterococci and 5 lactic group streptococci. These strains were examined for lipolytic activity when grown in skimmilk, cream, and skimmilk containing tributyrin; changes in proteolysis index, plate counts, and pH in skimmilk incubated at 7, 15, 21, and 32 C also were determined, And, combinations of enterococci and lactic streptococci were studied.

One-half of the Streptococcus durans strains frequently produced as much as 10 times more acetic acid than the others; the five strains of lactic streptococci consistently produced the lowest quantities of acetic acid. Compared with enterococci, except for Streptococcus faecalis var. liquefaciens, the lactic streptococci were more proteolytic, produced lower pH values, and had less viability at 15, 21, and 32 C. Enterococci other than S. faecalis var. liquefaciens were not proteolytic. All cultures showed tributyrinase activity; enterococci were the most active. Combining enterococci and lactic streptococci produced anomalous results.

Differential agar medium for separating Streptococcus lactis and Streptococcus cremoris

The characteristic ability of Streptococcus lactis and inability of Streptococcus cremoris to hydrolyze arginine formed the basis for the development of a differential agar medium to separate these species in pure and mixed cultures. Ammonia liberated from arginine was detected by the pH changes occurring in the medium. The agar contained milk as the sole source of carbohydrate, arginine as the specific substrate, diffusible (K(2)HPO(4)) and nondiffusible (CaCO(3)) buffer systems, and a suitable pH indicator in addition to other ingredients. The nondiffusible buffer system afforded the localization of pH changes, and, hence, the indicator color changes immediately around individual colonies appearing on the medium. S. cremoris produced yellow colonies surrounded by yellow zones on this purple medium because of their ability to produce acid from lactose in the milk. S. lactis, on the other hand, first produced colonies similar to S. cremoris, but subsequent color reversal of pH indicator with the liberation of NH(3) resulted in the discharge of the yellow color. Hence, S. lactis colonies were white and devoid of zones. The difference in their colony color allowed the identification of the species in a mixture of S. cremoris and S. lactis strains. The medium was found suitable for both qualitative and quantitative differentiation.

BACTERIAL TEST RESULTS OF GRADE-A RAW MILK SAMPLES AS A MEASURE OF FARM PRODUCTION CONDITIONS1

Milk from 30 grade-A farms was subjected to bacteriological tests including the Standard Plate, total, coliform, psychrophilic, thermoduric, and enterococcus count, resazurin reduction time, and leucocyte count to determine the correlation between these tests and farm production conditions. Farm conditions were evaluated at sample collection time by a farm score, which was based mainly on sanitation.

After collection and immediate transportation to the laboratory, half of each sample was stored at 3.3 C for 72 hr, the remainder was preincubated at 12.8 C for 18 hr after storage at 3.3 C for 54 hr; determinations were then performed. The leucocyte count was determined on the fresh sample.

The psychrophilic count was the only bacterial test that showed significant correlation with the farm score. For samples stored at 3.3 C for 72 hr, all comparisons among bacterial counts showed significant correlation except: psychrophilic count vs. resazurin reduction time; coliform count vs. resazurin reduction time; and, coliform count vs. enterococcus count. For preincubated samples, all comparisons among bacterial counts showed significant correlation except: psychrophilic count vs. resazurin reduction time; coliform count vs. resazurin reduction time; and, coliform count vs. thermoduric count. Higher correlations were obtained on the preincubated samples for all bacterial tests except the thermoduric count. Within this experimental design, preliminary incubation did not improve the ability of the bacterial tests to show statistically significant correlation with the farm score. The leucocyte count showed significant correlation with the farm score, but not with the bacterial test results.

Evaluation of data shows that the bacterial test results are not highly correlated with farm production conditions as measured by farm score. Milking-time inspections are necessary to assure that recommended practices are used in grade-A milk production.

An evaluation of the taxonomy of Propionibacterium

The object of this study was to reevaluate speciation of Propionibacterium by using numerical taxonomy. Fifty-six cultures representing eight species were studied. Thirty-eight morphological and physiological features were employed in the calculation of matching coefficients which were used to sort the cultures into groups. The results suggest a need for species consolidation; for example, Propionibacterium shermanii could appropriately become P. freudenreichii var. shermanii. Close resemblances were found between P. rubrum, P. peterssonii, and P. jensenii. Propionibacterium arabinosum and P. pentosaceum formed another mutually similar group. Further studies using additional cultural, physiological, serological, genetic, and phage host-range characteristics should be instituted to establish an improved classification of propionibacteria.

STARTER CULTURES FOR CHEDDAR CHEESE1,2,3

Starter bacteria play very important roles in the cheese vat including major contributions to flavor development and control of harmful microorganisms; hence, starter production is the single most important operation in Cheddar cheese production. Thus, the proper selection, blending, preservation, and packaging of starter strains by the culture suppliers and the adoption of strict sanitation, temperature control, and standardized procedures in cheese plants for bulk starter production would contribute toward ensuring a sound and unfailing starter program in the cheese industry.