International Journal of Food Micmhiology. 17 (1992) 101- i t I
101
© 1992 Elsevier Science Publishers B,V. All rights reserved 0168-1605/92/$05.110 FOOD 00552
Culture media for enterococci and group D-streptococci G. R e u t e r Institute for FrnTdHygiene. Mtat Hygiene and Technology. Veterinary Department, Free Utfil'eesityof Beeli~l, BeditJ. Germatty
Lanceficld group D-streptococci are contaminants of various food commodities, especially Ihose of animal origin. They encompass the new genus Enrerococcns comprising 13 known species and some species of streptococci which have their habitat in the intestine of animals, e.g. Strepu~.occus boris, s,is and equinus. The serologicaUy based grouping may no longer constitute the best definition for streptococci from the food chain. Food hygiene monitoring systems using enterococci as indicators need reliable methods for selective cultivation and identification of marker strains. Up to now more than 100 modifications of selective media have been described for isolating streptococci or enterococci from various specimens. The selection of a medium requires either experience or consultation. It depends on the kind of specimen, the method of cultivation (plate count or membrane tilter) and whether or not the habitat is heavily contaminated with other organisms. The choice of media is made more difficult as commercial versions of the same cullnre medium may vary in recipe a n d / o r performance from producer to producer. Therefore, reviewing the literature may help in the choice of medium and confirmation tests. The selectivity and productivity of some commonly used or cited media arc reported here, partly b a g d on our own experience: citrate azid¢ tween carbonate agar (CATC), kanamycin aesculin azide agar (KAA) and M-enterococcus agar (ME) including earlier results with ae~ulin bile azide agar (ABA), and thallous acetate telrazolium glucose agar (TIT@). No medium was completely selective for all group D-streptococci or for all enterococci but some media were highly selective for a singl~' Entemcoccus species, e.g,, for E- faecalis which ~rves as indicator of human pollution. Confirmatory tests mast be carried out when experience in the evaluation procedure is limited. Selective media for enterococci should be used only after or while checking in parallel their selectivity and productivity against appropriate lest organisms. Key words: Enterococci: Group D-streptococci: Cultivation; Identification: Selectivity; Productivity
lntrc~luction Enterococci may be considered an essential part of the autochthonous microflora of humans and animals. Some host specificity exists, in humans Enterococcusfaecalis and E. faecium are the most frequent species, while E. faecium is the predominant species in poultry and pigs. E. faecalis and faeciura as well as E. Correspondence ~ddress: G. Reuter, Bruemmerstrasse 10, D-1600 Berlin 33, Germany.
102
mundtii and casseliflacus can also be found in plants. This shows that there are some species which have their habitats in plants and insects. The ecology of enterococci needs further study especially on the basis of the new classification of the genus (Mundt, 1984). With this wide distribution it is not surprising that enterococci occur in different food commodities, especially those of animal origin. Presence of E. faecalis and faecium is often used to indicate faecal contamination of food. That means that these species must be cultivated separately from the total count of a food specimen. E. faecalis ssp. faecalis may be regarded as indicator of contamination from human sources, whereas E. faecium and other species may indicate contamination from other sources. Streptococcus bocis of the group D-streptococci, which occurs in animal faeces, is not generally suitable as a hygienic indicator as it cannot survive very well in the environment. S. equinus has so far not been isolated from humans and is probably of no interest for food hygiene. Therefore, in the field of food microbiology culture media for representatives of the new genus Enterococcus especially those for E. faecalis and its subspecies are most often needed. A wide variety of selective media for the enterococcus a n d / o r streptococcus group has been recommended and used in the past. More than 100 modifications have been described in the literature, summarized by Barnes (1976). Proposals for specific uses were made in the proceedings of a previous symposium (Reuter, 1985b). Some newer review data were given in the recent monograph of Devries¢ et al. (1991), but not all information related to food was included there. Taxonomic definitions must first be established to avoid confusion when using selective media or in interpreting results of counts of enterococci or streptococci. The last edition of Bergey's Manual (Mundt, 1984) does not contain all the new names of species which are accepted now (Devriese et al., 1991; Pompei et al., 1992).
Definitions to be considered in the past, the ecological grouping seemed to be a useful means of separation, e.g., into the 'faecal', 'oral' and 'lactic' steptococci. However, some overlapping existed. Both oral and lactic streptococci could occasionally be isolated from human faecal samples, sometimes even in large numbers (Barnes, 1976). Serological grouping was another well established way to divide the streptococci. There seemed to be certain correlations between ecological and serological grouping, for example, the faecal streptococci commonly corresponded to the group D-streptococci, the mesophilic lactic streptococci to the group N. The oral streptococci, however, could not be defined serologically as a single group. The new taxonomic separation into the genera 'Enterococcus' and 'Lactococcus' enables better identification of questionable isolates of streptococci: the outstanding problem is to separate the remaining D-positive strains from the enterococci easily. Streptococcus bocis, S, e.Tuinus a~;
101
© 1992 Elsevier Science Publishers B,V. All rights reserved 0168-1605/92/$05.110 FOOD 00552
Culture media for enterococci and group D-streptococci G. R e u t e r Institute for FrnTdHygiene. Mtat Hygiene and Technology. Veterinary Department, Free Utfil'eesityof Beeli~l, BeditJ. Germatty
Lanceficld group D-streptococci are contaminants of various food commodities, especially Ihose of animal origin. They encompass the new genus Enrerococcns comprising 13 known species and some species of streptococci which have their habitat in the intestine of animals, e.g. Strepu~.occus boris, s,is and equinus. The serologicaUy based grouping may no longer constitute the best definition for streptococci from the food chain. Food hygiene monitoring systems using enterococci as indicators need reliable methods for selective cultivation and identification of marker strains. Up to now more than 100 modifications of selective media have been described for isolating streptococci or enterococci from various specimens. The selection of a medium requires either experience or consultation. It depends on the kind of specimen, the method of cultivation (plate count or membrane tilter) and whether or not the habitat is heavily contaminated with other organisms. The choice of media is made more difficult as commercial versions of the same cullnre medium may vary in recipe a n d / o r performance from producer to producer. Therefore, reviewing the literature may help in the choice of medium and confirmation tests. The selectivity and productivity of some commonly used or cited media arc reported here, partly b a g d on our own experience: citrate azid¢ tween carbonate agar (CATC), kanamycin aesculin azide agar (KAA) and M-enterococcus agar (ME) including earlier results with ae~ulin bile azide agar (ABA), and thallous acetate telrazolium glucose agar (TIT@). No medium was completely selective for all group D-streptococci or for all enterococci but some media were highly selective for a singl~' Entemcoccus species, e.g,, for E- faecalis which ~rves as indicator of human pollution. Confirmatory tests mast be carried out when experience in the evaluation procedure is limited. Selective media for enterococci should be used only after or while checking in parallel their selectivity and productivity against appropriate lest organisms. Key words: Enterococci: Group D-streptococci: Cultivation; Identification: Selectivity; Productivity
lntrc~luction Enterococci may be considered an essential part of the autochthonous microflora of humans and animals. Some host specificity exists, in humans Enterococcusfaecalis and E. faecium are the most frequent species, while E. faecium is the predominant species in poultry and pigs. E. faecalis and faeciura as well as E. Correspondence ~ddress: G. Reuter, Bruemmerstrasse 10, D-1600 Berlin 33, Germany.
102
mundtii and casseliflacus can also be found in plants. This shows that there are some species which have their habitats in plants and insects. The ecology of enterococci needs further study especially on the basis of the new classification of the genus (Mundt, 1984). With this wide distribution it is not surprising that enterococci occur in different food commodities, especially those of animal origin. Presence of E. faecalis and faecium is often used to indicate faecal contamination of food. That means that these species must be cultivated separately from the total count of a food specimen. E. faecalis ssp. faecalis may be regarded as indicator of contamination from human sources, whereas E. faecium and other species may indicate contamination from other sources. Streptococcus bocis of the group D-streptococci, which occurs in animal faeces, is not generally suitable as a hygienic indicator as it cannot survive very well in the environment. S. equinus has so far not been isolated from humans and is probably of no interest for food hygiene. Therefore, in the field of food microbiology culture media for representatives of the new genus Enterococcus especially those for E. faecalis and its subspecies are most often needed. A wide variety of selective media for the enterococcus a n d / o r streptococcus group has been recommended and used in the past. More than 100 modifications have been described in the literature, summarized by Barnes (1976). Proposals for specific uses were made in the proceedings of a previous symposium (Reuter, 1985b). Some newer review data were given in the recent monograph of Devries¢ et al. (1991), but not all information related to food was included there. Taxonomic definitions must first be established to avoid confusion when using selective media or in interpreting results of counts of enterococci or streptococci. The last edition of Bergey's Manual (Mundt, 1984) does not contain all the new names of species which are accepted now (Devriese et al., 1991; Pompei et al., 1992).
Definitions to be considered in the past, the ecological grouping seemed to be a useful means of separation, e.g., into the 'faecal', 'oral' and 'lactic' steptococci. However, some overlapping existed. Both oral and lactic streptococci could occasionally be isolated from human faecal samples, sometimes even in large numbers (Barnes, 1976). Serological grouping was another well established way to divide the streptococci. There seemed to be certain correlations between ecological and serological grouping, for example, the faecal streptococci commonly corresponded to the group D-streptococci, the mesophilic lactic streptococci to the group N. The oral streptococci, however, could not be defined serologically as a single group. The new taxonomic separation into the genera 'Enterococcus' and 'Lactococcus' enables better identification of questionable isolates of streptococci: the outstanding problem is to separate the remaining D-positive strains from the enterococci easily. Streptococcus bocis, S, e.Tuinus a~;
