In a broader perspective it is invariably necessary to screen and examine a host of
pathological
specimens
, food, and pharmaceutical products (including dosage forms) to ascertain the presence of
staphylococci
; besides, specific organisms that are solely responsible for causing serious food contamination/
poisoning as well as systemic infections. There are
two media that are used extensively, such as :
(
a) Selective Media for Enterobacteria [or Enteric Bacteria]. [Greek. enterikos means pertaining
to intestine]. In general, all help in the degradation of sugars by means of the
Embden-Meyerhof
Pathway (or EMP-Cycle
] which ultimately cause cleavage of ensuing pyruvic acid to yield formic
acid in the formic-acid fermentations. It has been established beyond any reasonable doubt that in the
selective media
for enterobacteria a surface-active agent serves as the ‘main selector’, whereas in the
specific
staphylococcal medium the various selectors happen to be : sodium chloride [NaCl] and lithium
chloride [LiCl].
Staphylococci
are found to be tolerant against a ‘salt’ concentration extending ~ 7.5% (w/v)
e.g.
, Mannitol salt, Baired-Parker (BP), and Vogel-Johnson (VI) media.
Salient Features.
The other vital and important ‘salient features’ with respect to the various
other
‘principles’ concerning the selective media for staphylococci are as enumerated under :
(1) Use of a selective C-source
viz., mannitol or sodium pyruvate (soluble salt) along with a
suitable
‘buffer’.
(2) Use of an appropriate
acid-base indicator e.g., methyl-red phenolphthalein, for distinctly
visualizing the ensuing metabolic activity.
(3) By observing the ‘inference growth’.
(4)
Lecithin (a phospholipid) present in the egg yolk forms a vital ingredient of Baird-Parker
medium
seems to undergo hydrolysis strategically by the ensuing staphylococcal (i.e., esterase) activity*
in order that the prevailing microorganisms are adequately encircled by a
cleared (i.e., transparent)
zone
in the rest of the opaque medium.
B. Selective Media for Pseudomonads
Based on advanced, meticulous researches carried out on the
‘molecular analysis’,
pseudomonads
have been duly reclassified, and consequently several former Pseudomonas species
reallocated to
new genera, for instance : Burkholderia, Stenotrophomonas and others.
Importantly, these media solely depend upon the relative resistance of pseudomonas to the particular
quaternary ammonium disinfectant
cetrimide ; whereas, in certain recipes the incorporation of
nalidixic acid
i.e., an antibiotic, affords a reasonable resistance to the pseudomonads.
Laboratory Diagnosis.
The bacterium, Pseudomonads, usually grows rapidly on a plethora of
media thereby rendering the identification of the
pigmented strains of the organism from the clinical
samples rather easy. However, it has been duly observed that almost 1/10th of the isolates may be
nonpigmented.
[CH
3(CH2)15N(CH3)3]Br
Cetrimide
or
[Cetrimonium Bromide]
Interestingly,
two cardinal functionalities do confirm as well as ascertain the presence of the
Pseudomonads
, namely : (a) prompt oxidase reaction, and (b) arginine hydrolysis. A typical example
of such a media is as given below :
Cetrimide Agar Media [CA-Media].
It is usually employed to isolate the Pseudomonads from
either faeces or other specimens having mixed flora.
Special Note.
Because Ps. aeruginosa occurs as a most ‘frequent contaminant’, the actual
isolation of the ensuing bacillus from a
given sample must not always be accepted as a granted possible
proof of its
critical etiological involvement. Repeated isolation processes, therefore, may have to becarried out so as to help towards the actual confirmation for the prevailing diagnosis.