1. Why is it inappropriate to perform antimicrobial susceptibility testing on vi

Question

1. Why is it inappropriate to perform antimicrobial susceptibility testing on viridans streptococci (alpha hemolytic)isolated from a throat culture?

2. What Quality control do you run on a susceptibility test and how do you control the variables in the Kirby Bauer susceptibility test?

3. Describe the most reliable method to detect methicillin resistance in Staph aureus. Please explain how it works

4. Explain the D-zone test and how it is used for Staph aureus.

5. Explain how to test for Vancomycin resistance for Staph aureus.

Please Answer these my book is on its way im stuck ?

Explanation / Answer

1.Viridans streptococci represent a group of 24 currently described Streptococcus species that are nutritionally fastidious and mainly alpha-hemolytic on sheep blood agar. These gram-positive cocci are commensals of the oral cavity, upper airway, and gastrointestinal and genitourinary tracts. Despite the overall low virulence, they may cause infective endocarditis, contribute to polymicrobic abscess, and invade the bloodstream during the state of neutropenia.

It is inappropriate to perform antimicrobial susceptibility testing on viridans streptococci (alpha hemolytic)isolated from a throat culture because most of the strains develop antimicrobial and antibiotic resistance.Even if some sensitive colonies are found in plate but the number of resistant colonies are much more as compared to the sensitive ones.

2.The quality control one must run on a susceptibility test are as follows:

(a) Correct inoculum preparation.(b)Routine internal quality control testing with a range of control strains is a major part of the quality assurance process, as it facilitates monitoring of the performance of the test. Most standardized methods include tables of acceptable zone size ranges for control strains and, in addition to checking that control zone diameters are within the published ranges, rules or statistical approaches may be applied to indicate deviations from acceptable performance.(c)Internal quality assessment in which routine tests are repeated with the identity of the organisms blinded is a useful complementary approach to external quality assessment and may detect problem areas not highlighted by other control methods.(d)Education is an important part of the quality assurance process. Knowledge of atypical results for different organism-agent combinations may provide warning of possibly erroneous results, and an understanding of the limitations and sources of error in disc diffusion methods contributes significantly to the recognition, resolution and avoidance of errors.(e)Complete training of laboratory personnel.

The variables in the Kirby Bauer susceptibility test can be controlled by:

(a)Concentration of the antimicromial agent or antibiotic used.

(b)Incubation media

(c)Time of incubation and incubation temparature.

(d)The size of the zone of inhibition of growth is influenced by the depth of the agar.

3.In the absence of availability of molecular biology techniques, the cefoxitin disc was the
best predictor of methicillin resistance in S. aureus from among the techniques tested.

The validity of the disc diffusion can be evaluated by the test using cefoxitin (30mg), for this Mueller–Hinton agar plates were overlaid with an inoculum of the S. aureus clinical strains. Zone diameters were measured at 24 and 48 hours. S. aureus with a zone diameter of < 19mm were scored as resistant and those with a zone diameter of > 20mm were reported as susceptible.

4.The D-zone test detects inducible clindamycin resistance in Staphylococcus spp. D test should be used as a mandatory method in routine disc diffusion testing to detect inducible clindamycin resistance in Staphylococci for the optimum treatment of patients.The D-test needs to be performedat a shorter distance (15 mm) to prevent false-negative reporting.

Inducible resistance to clindamycin was tested by ‘D test’.Briefly, erythromycin (15 g) disc was placed at a distance of 15 mm (edge to edge) from clindamycin (2 g) disc on a Mueller-Hinton agar plate, previously inoculated with bacterial suspensions of Staph aureus. Following overnight incubation at 37°C, flattening of zone (D-shaped,Zone of inhibition) around clindamycin in the area between the two discs, indicated inducible clindamycin resistance.

5.Test for Vancomycin resistance for Staph aureus:

Mueller-Hinton agar plate was inoculated with bacterial suspensions of Staph aureus.Resistance to vanomycin was tested by placing a 30-g vanomycin disc on the agar plate following overnight incubation at 37°C.On initial testing of the S. aureus isolate after incubation,a small area of clearing was noted within a zone of reduced growth around a 30-g vancomycin disk, suggesting possible resistance towards vanomycin.

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