an epidemiological study of listeriosis in dairy cattle

addition to nalidixic acid. Lovett and others (1987) included cyclohexamide in the
FDA (United States Food and Drug Administration) broth to inhibit yeast and moulds.
This original formulation of Lovett and others (1987) was further developed by USDA
(United States Department of Agriculture) to trace Listeria in meat samples (McClain
and Lee 1988).
In parallel to the development of selective enrichment broths, several selective
agars were developed incorporating the same antibiotics. Gray and colleagues (1950)
were again the first researchers to use selective agar by including potassium tellurate
to inhibit Gram negative organisms. McBride and Girard (1960) developed a selective
phenyl ethanol agar containing lithium chloride, glycine and blood. This was the only
commercially available selective agar for a considerable time. A major problem was
its inability to inhibit enterococci. Lee and McClain (1986) later modified McBride
agar by removing glycine and blood, adding moxalactam and glycine anhydride and
increasing the concentration of lithium chloride. Ralovich and colleagues (1971) also
used trypaflavin and nalidixic acid in a serum based agar.
The discovery that Listeria organisms were resistant to higher concentrations of
lithium chloride led to development and modification of many agars and overcame the
problem of enterococcal growth. Most of the agars currently used are a modification of
McBride (McBride and Girard 1960) and the Oxford formulation (Curtis and others
1989a). In addition to antibiotics, one or more indicators for Listeria were also
included in the medium such as aesculin in Oxford agar (Curtis and others 1989),
blood in enhanced haemolysis agar (EHA, Cox and others 1991) and Fenlon Listeria
agar (Fenlon 1985), mannitol and phenol red in PALCAM agar (van Netten and others
1989) and rhamnose in Modified Despeirres agar (Golden and other 1988).
Although the selectivity of media was increased by incorporating antibiotics in the
culture medium it was noticed that highly selective media could also inhibit the
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