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close this bookBasic Laboratory Procedures in Clinical Bacteriology (WHO; 1991; 128 pages)
View the documentPreface
View the documentIntroduction
open this folder and view contentsQuality assurance in microbiology
close this folderPart I. Bacteriological investigations
open this folder and view contentsBlood
close this folderCerebrospinal fluid
View the documentIntroduction
View the documentCollection and transportation of specimens
View the documentMacroscopic inspection
View the documentMicroscopic examination
View the documentPreliminary identification
View the documentSusceptibility testing
open this folder and view contentsUrine
open this folder and view contentsStool
open this folder and view contentsLower respiratory tract infections
open this folder and view contentsUpper respiratory tract infections
open this folder and view contentsSexually transmitted diseases
open this folder and view contentsPurulent exudates, wounds, and abscesses
open this folder and view contentsAnaerobic bacteriology
open this folder and view contentsAntimicrobial susceptibility testing
open this folder and view contentsPart II. Essential media and reagents for isolation and identification of clinical pathogens
View the documentSelected further reading
View the documentSelected WHO publications of related interest
View the documentBack Cover
 

Preliminary identification

Growth on MacConkey agar is suggestive of Enterobacteriaceae and should be further identified using the methods and media recommended for enteric pathogens.

Colonies of Gram-positive cocci with a narrow zone of beta-haemolysis may be S. agalactiae (group B streptococci). This should be confirmed with the CAMP test.

Flat colonies with a concave centre and a slight zone of green (alpha) haemolysis are probably S. pneumoniae. For confirmation, a 6-mm optochin disc should be placed on a blood agar plate heavily inoculated with a pure culture of the suspected strain. After overnight incubation, pneumococci will exhibit an inhibition zone of more than 14 mm around the optochin disc. If the reading of this test on the primary blood agar plate is inconclusive, the test should be repeated on a subculture.

Colonies of H. influenzae will grow only on chocolate agar, and as satellite colonies in the vicinity of the staphylococcal streak on blood agar. Further identification may be accomplished using H. influenzae type b antiserum in the slide agglutination test.

Table 7. Choice of culture media for CSF specimens according to the results of the Gram smeara

Observation

Gram-negative rods in neonates

Gram-negative rods, other ages

Gram-positive cocci in neonates

Gram-positive cocci, other ages

Gram-negative cocci

Gram-positive rods

No organisms seen

Blood agarb

+

+

+

+ with optochin disc

+

+

+

Blood agar with Staphylococcus aureus streakb

+

+

       

+

Chocolate agarb

 

(+)

   

(+)

 

(+)

MacConkey agar

+

(+)

         

TSB

+

+

+

+

+

+

+

 

a + = use; (+) = optional use.

b Incubate in an atmosphere rich in CO2 (candle-jar)

Gram-negative diplococci growing on blood and chocolate agar, and giving a rapidly positive oxidase test, may be considered to be meningococci. Confirmation is accomplished by grouping with appropriate N. meningitidis antisera (A, B, C) in the slide agglutination test. A negative agglutination test does not rule out meningococci as there are at least four additional serogroups. If the agglutination test is negative, carbohydrate utilization tests should be performed and the culture sent to a central reference laboratory. A preliminary report should be given to the physician at each stage of identification (Gram-stain, growth, agglutination, etc.), noting that a final report will follow.

Colonies of Gram-positive rods with a narrow zone of β-haemolysis on blood agar may be Listeria monocytogenes. The following confirmatory tests are suggested: catalase positivity, motility in broth culture or in MIU, growth and black discoloration on bile-aesculin agar.

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