Image Article
Alternate and non-staining methods for confirming cell
wall of Gram indeterminate bacteria
Venkataramana Kandi
Address for Correspondence: Dr.Venkataramana Kandi, Assistant Professor, Department of Microbiology,
Prathima Institute of Medical Sciences, Karimnagar, Telanagana, India.
email: ramana_20021@rediffmail.com
Clinical microbiologists during their laboratory practice are
required to isolate and identify the bacteria from various
clinical samples. In most instances gram’s stain is sufficient
to know if a bacterium possesses a gram positive cell wall or
a gram negative cell wall. Some bacteria produce rough
colonies which are hard to emulsify (eg: Actinomyces spp,
Corynebacterium spp, propionibacterium spp) and do not
reveal the exact gram’s reaction. Few other bacteria are
inherently gram variable and pose difficulty in identifying
their gram’s reaction. Bacillus spp, Butyrivibrio, and
Clostridium spp are gram positive bacteria which show gram
negative forms misleading their identification by
microbiologists.
Studies in the past have evaluated the alternate and nonstaining methods to determine if the isolated bacterium has
a gram positive or a gram negative cell wall. KOH string test
and the antibiotic disc test (vancomycin 5 μg and colistin 10
μl) are the alternates to grams stain where in gram positive
bacteria are KOH string test negative, vancomycin 5 μg –
sensitive and colistin 10 μg-resistant. Gram negative bacteria
are positive for KOH string test, vancomycin 5 μg -resistant
and colistin 10 μg –sensitive. For KOH test the bacterial colony
is picked up with the help of an inoculating wire loop and is
emulsified in a drop/10 μl 3% KOH on a clean and grease
free slide1,2. After stirring/mixing the colonies for about 60
seconds we should look for the appearance of viscous nature
and in that case the loop should be raised very slowly to see
the formation of stringiness indicating that it is a gram
negative bacterium. Gram positive bacteria don’t form
viscous emulsion and are negative for string like formation
as shown in Figure.
REFERENCES
1.
Buck JD: Nonstaining (KOH) method for determination
of gram reactions of marine bacteria. Applied and
Environmental Microbiology. 1982, 44, 992-993.
2.
Halebian S, Harris B, Finegold SM, Rolfe RD: Rapid
method that aids in distinguishing Gram-positive from
Gram-negative anaerobic bacteria. Journal of Clinical
Microbiology. 1981, 13, 444-448.
39
Demonstration of string test for the determination of bacterial
cell wall type (gram positive/gram negative cell wall)
Please cite this article as: Venkataramana Kandi. Alternate
and non-staining methods for confirming cell wall of Gram
indeterminate bacteria.Perspectives in medical research
2015;3:2:39.
Sources of Support: Nil,Conflict of interest:None declared