Question

what are the disadvantages and advantages of biochemical tests for bacterual identifications please explain( at least a paragraph)

some if the test include the glucose iron agar test, sucrose test, lactose test, nitrate reduction test, oxidase test, catalase test, DNase test, gelatin test.

Answer 1

Answer :

Considering the vast number of microorganisms in the environment around us and also considering the bacteria that are present inside our gut and on our surface which are either commensals (harmless), pathogens (harmful) or opportunists (harmful only in certain conditions). It is mandatory that we should have some way of identifying these bacteria in order to protect ourselves from the harm by prevention or by treatment.

The biochemical tests for identification were developed with above consideration and knowing very well that these bacteria have very different reactions when exposed to certain substrates and depending on their enzyme activities they give varied reactions.

Advantages

• Help in identification of pathogens
• Help in understanding the pathogenicity and virulence of the pathogenic bacteria
• Also helps in classifying bacteria into relevant groups depending on their biochemical characteristics and hence make study of bacteria easy
• With advances in automation in microbiology, it has become possible to carry out a large number of biochemical tests in a small time and space thereby making identification process easier and faster. For example - API system of identification and others.

Disadvantages

• The continuous changes in bacterial profile by mutation or by acquiring certain traits at times makes it difficult to properly identify bacteria
• Overlapping of biochemical reactions within certain group of bacteria such as Enterobacteriaceae leads to mistakes in identification thereby defeating the whole purpose
• Biochemical tests consume lot of resources making them expensive and rather impossible for any lab to carry out the full array of tests. This leads to carrying out very few tests and literally guessing the identification.
• Biochemical tests need overnight incubation and thus takes lot of time to reach a positive identification.
• In the wake of newer methods based on genetic and molecular biology the identification process is getting revolutionized and thus making biochemical tests obsolete.

Given below are few biochemical tests commonly used and how they help in identification :

1. Glucose Iron agar test - This test identifies whether bacteria ferment sugar and also produce hydrogen sulphide which reduces Iron to Iron sulphide. The fermentation of glucose produces acid and/or gas seen by change in media color by using indicators and iron sulphide gives a black deposit. This test combines more than one parameter and eases identification. For example - Salmonella typhi will give both test positive.
2. Sucrose test - This test identifies bacteria which can hydrolyze sucrose to glucose and fructose. Not all bacteria can carry out this hydrolysis. For example - Proteus vulgaris
3. Lactose test - This is a very simple, commonly applied test in primary cultures to identify bacteria into Lactose fermenters - Escherichia coli, Klebsiella pneumoniae, etc and Lactose non-fermenters - Salmonella typhi, Shigella species, etc.
4. Nitrate reduction test - This test though not commonly employed is used as a secondary biochemical test to identify bacteria who can reduce nitrates to nitrites. For example - differentiating Neisseria gonorrhoea which does not reduce nitrates from similar bacteria such as Moraxella catarrhalis and Kingella denitrificans which reduce nitrates.
5. Oxidase test - helps in identifying those bacteria which produce cytochrome c oxidases enzymes.For example - Pseudomonas aeruginosa
6. Catalase test - The ability of bacteria to breakdown Hydrogen peroxide producing water and oxygen is tested by this rapid bench side test. Very helpful in differentiating Staphylococci (Catalase +ve) from Streptococci (Catalase -ve)
7. DNase test - Identifies those bacteria which can hydrolyze DNA molecule and use it as a source of Carbon and energy. This again is an advanced biochemical test done only after presumptive identification and used as confirmatory test. For example - Campylobacter jejuni , Serratia marcescens
8. Gelatin test - Identifies those bacteria that liquefy gelatin by hydrolyzing it. For example - Bacillus subtilis, Clostridium tetani