Medical Microbiology(Cont')

This is Jennifer from TG02. As what Muna had mentioned on the previous post, we were posted to CPA( Central Processing Area) in the Department of Bacteriology. I was rotated to the repiratory section during the third week of my SIP where i recieved specimens from patients from their upper or lower respiratory tract.

( Very sorry that due to some problems, i couldnt upload the pictures that i have downloaded. I will upload the pictures as soon as i have settled the problem.)


Upper respiratory tract infection

MRSA (Methicilin Resistant Staphylococcus aureus) Screening
MRSA can also be known as multidrug resistant Staphylococcus aureus as it is resistant to a huge group of antibiotics known as beta- lactams. This bacteria cause many infections which are difficult to treat since they are also resistant to cephalosporin and penicilins.

Usually, most of the samples that are taken to detect for the presence of MRSA are nasal swab. Screening for MRSA requires the use of BHIB (Brain Heart Infusion Broth) and MRSA agar which is a selective agar that only allows the growth and differentiate MRSA from other bacteria. BHIB contain a type of antibiotic called oxicilin and this antibiotic will kill most of the bacteria in the nasal swab except for MRSA as MRSA can withstand this antibiotic and is an enriched broth for staphylococcus species.

MRSA selective agar BHIB

Procedures
1. Incubate the cotton bud that contain the nasal swab into BHIB and incubate for 24 hours to give enough time for the antibiotic to kill off most of the other types of bacteria and allow MRSA to grow, in an oxygen incubator.
2. After 24 hours incubation, sub the cotton bud which was incubated in the BHIB on MRSA selective agar and do normal agar streak.
3. Incubate the agar in an incubator for 2days for the bacteria to grow.

Results
If MRSA is present and grow on the agar, the positive result is shown a purple colony growing on the agar which is shown on the diagram below.

Purple colonies growing on MRSA agar

Lower Respiratory Tract Infection

Sputum Culture
Usually, sputum samples are obtained from patients to detect for the presence of lower respiratory Tract Infection. The appearance of sputum can be watery, clear or mucoid. Before doing sputum culture, we have to do a gram stain for the sputum first as we need to identify whether the sputum sample is a good sample to do the culture or not.

After viewing the gram stain of sputum under the microscope, it can be determine whether the remaining sputum sample can be carried on to do the culture or should it be rejected.
A good sputum sample should have absence of epithelial cells and presence of PMN (Polymorph nuclear leukocyte) as presence of high number of epithelial cells shows that the sample obtained could be from saliva as saliva sometimes does contain epithelial cells from our cheeks. High number of PMN shows that the sample is as good sample as it shows that there is inflammation by the infections caused by pathogens.

Other types of samples such as CSF (Cerebrospinal Fluid) and ETTA (Endotracheal Tube Aspirate) also requires gram staining and cultures. The only difference is that these two types of samples do not require doing gram stain first before culture can be done. Both the culture and gram stain can be done at the same time.

Below is a tabulated table showing which agar or broth to use for each of the type of samples.

(Staphylococcus streak is done on all blood plates after normal streaking for any type of samples as staphylococcus provide a particular essential nutrient for the bacteria on the blood agar to grow.)

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Procedures (For sputum)
1. The appearance of the sputum was checked. (Such as whether the sputum is mucoid, watery, clear, or purulent.)
2. Using a labeled slide, a small circle was drawn on the under side of the slide.
3. The surface of the slide was swab by alcohol swab.
4. Using a cotton bud, the sputum was stirred and a small amount of sputum was obtained.
5. A small amount of sputum was applied onto the slide between the circles that was drawn on the underside.
6. The sputum smear was fixed by putting the slide on the heater until smear was dried.
7. Gram stain was performed.
8. The slide was viewed under the microscope.

If the sample is a good sample, carry on with sputum culture. If the sample is not good sample, reject it.
For a good specimen of sputum, there should be no or minimal epithelial cells seen and PMN (Poly morphs nuclear). If too many epithelial cells are seen, it could mean that the specimen is the saliva of patient not the sputum as saliva may contain epithelial cells that are actually the cheek cells. Presence of PMNs show that the is inflammation of the patient’s lower respiratory tract as pathogens are present and cause the activation of PMNs.
Cont’
9. The remaining sputum was subbed onto Mackonkey and Blood agar.
10. Normal streaking was done.
11. Staphylococcus streak was done on blood agar.


References:
MRSA Screening: The Test. Retrieved from 12 July 2009 from Lab Test Online: http://www.labtestsonline.org/understanding/analytes/mrsa/test.html