Page 163 - Review of Medical Microbiology and Immunology ( PDFDrive )

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mebooksfree.com mebooksfree.com mebooksfree.com is fermented, both the butt and the slant will be red. The mebooksfree.com
PART II Clinical Bacteriology
152
AGAR MEDIA FOR ENTERIC GRAM-NEGATIVE RODS
Triple Sugar Iron (TSI) Agar
slant can become a deeper red-purple (more alkaline) as a
The important components of this medium are ferrous sulfate
and the three sugars glucose, lactose, and sucrose. Glucose is
deamination of amino acids. (4) If H S is produced, the
present in one-tenth the concentration of the other two sugars.
2
black color of ferrous sulfide is seen.
The medium in the tube has a solid, poorly oxygenated area on result of the production of ammonia from the oxidative
The reactions of some of the important organisms are
the bottom, called the butt, and an angled, well-oxygenated
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presented in Table 18–7. Because several organisms can give
area on top, called the slant. The organism is inoculated into
the same reaction, TSI agar is only a screening device.
the butt and across the surface of the slant.
The interpretation of the test results is as follows: (1) If
lactose (or sucrose) is fermented, a large amount of acid is
The import ant components of this medium are urea and
produced, which turns the phenol red indicator yellow both
the pH indicator phenol red. If the organism produces ure-
in the butt and on the slant. Some organisms generate gases,
ase, the urea is hydrolyzed to NH and CO . Ammonia
which produce bubbles in the butt. (2) If lactose is not fer-
turns the medium alkaline, and the color of the phenol red
mented but the small amount of glucose is, the oxygen-
deficient butt will be yellow, but on the slant, the acid will be
organisms that are urease-positive are Proteus species and
oxidized to CO and H O by the organism and the slant will
2
2
K. pneumoniae.
be red (neutral or alkaline). (3) If neither lactose nor glucose changes from light orange to reddish purple. The important
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often sufficient to identify the genus of an organism;
however, an array of 20 or more biochemical tests is
4/dL in municipal drinking water is indicative of unac-
ceptable fecal contamination. Because E. coli and the
required to identify the species.
Another valuable piece of information used to identify
some of these organisms is their motility, which is depen-
ing water, there is rarely a problem with meeting this
dent on the presence of flagella. Proteus species are very
standard. Disinfection of the public water supply is one of
motile and characteristically swarm over the blood agar enteric pathogens are killed by chlorination of the drink-
the most important advances of public health in the twen-
plate, obscuring the colonies of other organisms. Motility is
tieth century.
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also an important diagnostic criterion in the differentiation
Antibiotic Therapy
of Enterobacter cloacae, which is motile, from K. pneu-
moniae, which is nonmotile.
If the results of the screening tests suggest the presence
bers of the Enterobacteriaceae and related organisms must
of a Salmonella or Shigella strain, an agglutination test can
be individually tailored to the antibiotic sensitivity of the
be used to identify the genus of the organism and to deter-
organism. Generally speaking, a wide range of antimicrobial
mine whether it is a member of group A, B, C, or D.
agents are potentially effective (e.g., some penicillins and
cephalosporins, aminoglycosides, chloramphenicol, tetra-
Coliforms & Public Health
usually depends on the results of antibiotic sensitivity tests.
Contamination of the public water supply system by sew-
Note that many isolates of these enteric gram-
age is detected by the presence of coliforms in the water. In
a general sense, the term coliform includes not only E. coli, cyclines, quinolones, and sulfonamides). The specific choice
negative rods are highly antibiotic resistant because of
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the production of β-lactamases and other drug-modifying
but also other inhabitants of the colon such as Enterobacter
and Klebsiella. However, because only E. coli is exclusively
enzymes. These organisms undergo conjugation fre-
a large intestine organism, whereas the others are found in
quently, at which time they acquire plasmids (R factors)
the environment also, it is used as the indicator of fecal
contamination. In water quality testing, E. coli is identified
plasmid-encoded New Delhi metallo-β-lactamase causes
by its ability to ferment lactose with the production of acid
resistance to penicillins, cephalosporins, monobactams,
and gas, its ability to grow at 44.5°C, and its characteristic
and carbapenems.
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