133 exposed to the extra-enteral conditions, their isolation on selective

media discriminates against those that have, stress factors apart, lost the ability to perfo3?m a function such as the fermentation of lactose or the reduction of tétrazolium chloride. Consequently,

the true density of such bacteria in water cannot be accurately measured. For these reasons, colony counts appear to have no absolute significance but serve as an indication of the number of organisms able to form colonies at that time on some artificial medium.

4.2. Model Media

Media for increased recovery of organisms have been proposed. To achieve such a medium Dufour and Oabelli (1974) tested a variety of fermentable carbohydrates, and lactose was found to be the most effective for the differentiation of coliforms. It is not surprising that despite the suggestions of Mackie (1913) and of Mossel (1957) for the replacement of lactose with mannitol, lactose still remains the sugar of choice in the primary isolation medium for coliforms. The major advantage of lactose over mannitol is that pathogens can be generally differentiated from non-pathogens by their lactose reaction

(although lactose-negative non-pathogens are known).

The ideal medium should have the attribute of permitting the recovery of the same number of both stressed and non-stressed organisms that would grow on a non-selective or enriched medium. This ideal medium should also be specific enough to recover only the desired organisms, selective enough for their total recovery and finally, it should inhibit the

background overgrowth of other unwanted organisms. So. far no ideal selective medium is available that would fit the above criteria.

Attempts at achieving this ideal status have been made recently, notably by Dufour and Oabelli (l974) for coliform isolation and by

13%.

Abshire (197?) and by Donnelly and Hartman (l97S) for group D streptococci isolation. The odd choice of some of the constituents of these media is one drawback, the other is the difficulty of acceptance of such media into widespread use so as to replace the well-known and time-tested media currently used in water bacteriology.

It is reasonable to be aware of the presence of 'stressed’ or "injured" bacteria in water. These organisms do not deserve too much emphasis in practical water bacteriology. They are usually outnumbered by viable organisms and contribute very little to indicate hazardous contamination of water. Moreover, they are not the last bastion of water quality. What constitutes real danger to public health is the presence in water of living bacteria not dead or ’stressed’ cells.

However, stressed cells, although they may not grow on sélective media, may be quite viable as pathogens in the animal body. As a recognition of the presence of'*stressed''cells a resuscitation period of incubation is usually employed. This period conditions these^stressed'cells for growf'bh on the selective media.

4.3# Selectivity of Media

Slanetz and Bartley’s medium has been reported as being very selective for group D streptococci, and when incubated at an elevated temperature (44° - 45°û) all red or maroon colonies may be accepted as faecal streptococci (Taylor and Burman, 1964| Mead, 1966; D.H.S.S.

(Welsh Office), 1969). Comparison of KF streptococcus agar with this medium on water and sewage samples show the superiority of Slanetz and Bartley’s medium over KF. Eaibaud et , (1961 ), on the contrary, observed that Slanetz and Bartley’s medium was not totally selective

for group D streptococci as lactobaoilli overgrew the group D streptococci in their study of pig caecal samples, Pavlova et al., (l9?2), in their

135.

study of group D streptococci in faeces, sewage and food samples, also reported that 18.4/^ of the organisms growing on this medium were not group D streptococci. More recently, Burman et (l978) and Noble,

(1978) claimed that this medium was not 100^ selective for the group D streptococci. It is interesting to note that Noble, (l97S) incubated his plates at 37°C. Had Noble incubated his plates at the recommended temperature, one would have supposed that more group D streptococci would have been isolated. The disadvantage of this medium it has been claimed, is the inhibitory effect of the azide on S. bovis (Sabbaj et al., 1971; Switzer and Evans, 1974). The results from this study indicate that the selectivity of the medium was indeed specific. Although the percentage

recovered was low, this may be due to their rapid die-off rate. McFetters et al., (l974), have shown that 8.bovis and 8. equinus die-off faster than fgiecalis and 8. faecium. Furthermore, none of the group D streptococci strains failed to tolerate 40^ bile + 0.1^ aesculin»

The reliability of METB, on the other hand, has not been subjected to wide criticism provided the recommended standard grade of Teepol 610 (b.D.H,) is used. An improvement could be obtained by gelling the medium with purified agar » The advantage of this is that colonies developing on the agar are larger and more robust than those on absorbant pad

(personal observation). In addition, lactose-fermentation characteristics appear to be unequivocal on the agar medium. -.4. Spéciation of Coliform Isolates

The primary task for the early water bacteriologists had been in the identification of coli sensu strie tu, by using the Eijkman test and indole production at 44°0. The standard IMVrC tests (Parr, 1938) had been employed as. an extension of this for the identification of other coliforms. Other workers examining coliforms from environmental sources have not speciated their isolates beyond the determination of the

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