Hi everyone, it’s my turn again this week. For this week, I think I’m going to share about what I’ve learned from the microbiology section from my lab. I’m going to introduce this kit that can help to identify the different types of gram-negative bacteria. It is named the Microbact Gram-Negative Identification System.
Principle:
The Microbact Gram-negative system is a standardized micro-substrate system designed to stimulate conventional biochemical substrates used for identification of Enterobacteriaceae and common miscellaneous Gram-negative bacilli. Organism identification is based on pH change and substrate utilizations.
This product consists of two separate substrate strips, 12A and 12B. Each strip consists of twelve different biochemical substrate. The 12A strip can be used alone to identify oxidase-negative, nitrate positive glucose fermenters and can be used for screening pathogenic Enterobacteriaceae from enteric and urine specimens or identification of other common isolates. The 12B strip can be used together with the 12A strip for identification of oxidase positive, nitrate-negative, glucose non-fermenters and the Enterobacteriaceae.
Each kit contains one holding tray for the 12A and 12B strips and organism ID report forms including colour interpretation chart.
Examples of species of oxidase negative, Gram-negative bacilli that can be identified using 12A strip alone:
Acinetobacter spp., Shigella spp., Enterobacter spp., Escherichia spp., Salmonella spp., etc.
Examples of species of Gram-positive bacteria that can be identified by using combination of 12A and 12B strips:
Vibrio spp., Pseudomonas spp., Pasteurella., etc.
Set up procedure:
1.Isolation
To use the kit for identification of organism, a pure culture of that particular organism must be obtained. Appropriate agar media such as MacConkey, Blood or Chocolate may be used to grow the organism for approximately 18 to 24 hour. I believed you all would know how to do a pure culture plate by now so I won’t state the procedures here.
Before using the kit, an Oxidase test should be performed on that organism to be identified. As mentioned above, if it’s positive, 12A and 12B strips should be used and if it’s negative, 12A strips can be used alone.
2.Preparation of inoculum
Pick 1-3 isolated pure colonies from the 18-24hr culture and emulsify in 2.5 to 5 ml of sterile saline. Usually 1 colony is enough to emulsify in 2.5ml of saline if doing on the 12A strip alone. If it’s doing 12A and 12B strips, it may need 2 colonies to emulsify in 5ml of the saline. Then, mix thoroughly for a uniform suspension.
3.Inoculation
As the strips are sealed, the individual substrate sets can be exposed by cutting the sealing strip and slowly peel it back. Then, place the strip on the holding tray and using a sterile dropper, fill the bacterial suspension to half of each well in the set. After that, drop one drop of sterile mineral oil on the wells that have the substrates underlined (for 12A strip is well 1,2 and 3 while for 12B strip is well 8 and 12).
In case you all want to ask me what each well contains, here’s the answer:
For 12A strips-Well 1- Lysine
Well 2- Ornithine
Well 3- H2S
Well 4- Glucose
Well 5- Mannitol
Well 6- Xylose
Well 7- ONPG
Well 8- Indole
Well 9- Urease
Well 10- VP
Well 11- Citrate
Well 12- TDA
For 12B strips-Well 1- Gelatin
Well 2- Malonate
Well 3- Inositol
Well 4- Sorbitol
Well 5- Rhamnose
Well 6- Sucrose
Well 7- Lactose
Well 8- Arabinose
Well 9- Adonitol
Well 10- Raffinose
Well 11- Salicin
Well 12- Arginine
4.Incubation
Reseal the inoculated rows with adhesive seal and write the specimen identification no.(in my lab case is using barcode no.) on the seal with a marker. Then, incubate 33-37°C for 18 to 24hrs.
5.Reading the test strips
Remove the test strips from the incubator and unseal them for reading. Positive and negative reaction results are recorded down on the organism ID report forms by comparing the colour change with the colour chart. For some of the wells, there’s an additional reagent to add to before reading. For 12A strips, well 8 drop 2 drops of Indole (Kovacs) reagent and read within 2 minutes, well 10 add 1 drop of VPI and 1 drop of VPII and read within 15 to 30 minutes and well 12 add 1 drop of TDA reagent and read immediately. These additional reagents are required for the reaction to start as different bacteria can have different reaction to these reagents.
There’s also an additional test called the Nitrate Reduction Test. It is performed in well 7 (ONPG) after reading the ONPG reaction. 1 drop of Nitrate reagent A and 1 drop of Nitrate reagent B is added to the well and a change to colour red within few minutes indicates nitrate reduction to nitrite. The positive reaction indicates that it is an organism belonging to the family of Enterobacteriaceae.
6.Interpretation
For the interpretation is pretty easy as they adopted a coding system/program that interpret the type of bacteria simple by entering the recorded result. For every 3 reaction (every three wells), it is added up to a single digit of the code. So if only 12A strip is used, there will be 4 digits which make up the code. If using 12A and 12B strips together, there will be a total of 8 digits that make up the code. Simply just choose the correct program and enter the code and the system will tell you what kind of bacteria is it.
Alright, that's it for this week. Enjoy your life people, if you still can now. Lol.
Sharon
Tg01
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10 comments:
Hi Sharon,
Haven seen you for quite some time. Haha. This method is the one Dr Alex mentioned before right?
Is there any disadvantage to using this kit? Other than cost?
Thanks :)
Ying Chee
TG01
Hi..
would like to know when is it required to identify the gram-negative bacteria?you dont possibly do this test on ALL gram-negative bacteria found rite?
thanks =)
Nur Farhana
0604834B
Hello sharon!
you mention: Appropriate agar media such as MacConkey, Blood or Chocolate... ...
did i see wrongly? i never heard before chocolate agar media. can briefly tell me what type of microorganism is best grown on chocolate?
the way the organism is identifed is acording to their characteristic right? (as in substrate utilization) is there possibility that there are 2 species of organism with the same characteristic?
are there times when you cannot identify the organism using that kit? if yes, then what do you do?
Thanks,
Yu Mei
TG01
hey Sharon,
You mentioned about the interpretation system rite...
about the coding part...
Does that mean that every time you get a result u will have a code?
Juz explain briefly can...hehe
thanks
Neela
TG02
HELLO SHARON,
I just wanna know how you know that it is positive in each well. Then if it is not a gram-negative bacteria, how you all identify them?
LESLIE (:
To Ying Chee:
This kit is different from what Mr. Alex showed us before but it’s somehow similar idea which is to confirm the species of bacteria. There are some limitations to this kit. It may not be able to identify all types of bacteria as some bacterial strains may have biochemical reactions due to unusual nutritional requirements or mutations making it difficult to be identified. Reactions obtained using this system may be different from published results which uses other substrate formulations. Prolonged incubation, improper filling of wells, or inadequate inoculum may lead to false result.
To Farhana:
Sorry Farhana, I’m not too sure about this. I’ll try to find out but you might need to wait for 2 weeks because now I’m in a different branch and this test is only perform in the main lab and I’m only going back after 2 weeks then I can clarify with the staffs there.
To Yu Mei:
Chocolate agar plate is not made of chocolate. It’s just that the colour is brown. It is a type of blood agar plate which the blood cells has been lysed by heating the cells to 56°C. It’s a non-selective agar that is used to grow fastidious respiratory bacteria. As for the characteristics of different bacteria, some bacteria may have some same characteristics with other bacteria but there will sure be a slight difference in their characteristics no matter what. There will not be exactly same characteristics unless it’s the same kind of bacteria. That’s why the kit is designed to have so many different kind of substrate. And ya sometimes it might happen that the kit is not able to identify certain organism as some bacterial strains may have atypical biochemical reactions due to unusual nutritional requirements or mutations which makes it very difficult to be identified. If such cases happen, they would have to do another test instead of using this kit to identify.
To Neela:
Yup, for every test, there will be a code that belongs to certain species under the program. For every well, there’s a digit so if it’s positive you will take the digit to add with the other wells that have positive reaction too and add up. If it’s negative it will be recorded as zero. The digits are added up for every 3 wells. As for each strip there’s only 12 wells, there will be a total of 4 digits that will form the code. You just have to enter the code into the computer system and they will help you interpret what bacteria it is.
To Leslie:
The reaction is compared with a colour chart as I have mentioned in my posting. The colour chart will show one positive and one negative reaction colour of that well look like. The very first thing to do when deciding whether to use this kit is to do a gram stain. If it’s gram-negative bacteria then we will use this kit. But if it’s gram-positive bacteria, there’s another test to perform.
hey.
i have one question.
why is the 12B kit used only wen the Oxidase test is positive?
sutiana
Hi Sharon!
I've been posted to a mircrobiology lab just starting this week(sept 1st) and we're using the exact same thing that you mentioned in the blog(the 12and 12B strips). There, we use it for confirmatory tests too for the identity of a particular microorganism. However, some (really) minor differences in the procedure is that we use a syringe and needle instead of a dropper to fill the wells with the respective microorganisms emulsion. Also, we would conduct sensitivity tests to various antibiotics alongside the identity test so that when the identity is out, we would be able to determine which antibiotics would be suitable against the particular microbes too.
Thanks there!
Alexander Soo TG02
0608122H
To sutiana:
12A strip itself can’t identify the oxidase-positive bacteria. It needs 12B strip to go along with it. So they are using 12A and 12B together not 12B only.
To Alexander:
Oh ya they do conduct sensitivity test too. And the reason for using dropper for the company I’ve attached to is most probably because of the difference in cost.
Back to Farhana,
The test kit is used for any gram-negative bacteria. Only gram-positive bacteria is not used for this kit.
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