Biochemical tests are tests that help in the identification and differentiation of bacteria based on their metabolic activities. Each bacteria utilizes certain compounds (such as carbohydrate, protein, organic compounds) depending upon the availability of enzymes or appropriate environmental conditions (availability or lack of oxygen).
The end product of such reactions can be detected either by pH changes or by using indicator chemicals. The end results of such metabolic reactions form the basis of identification and differentiation of bacteria either to genus or up to species level.
BioMérieux's API identification products are test kits for the identification of Gram-positive bacteria, Gram-negative bacteria, and yeast.
Bacterial species capable of producing H₂S release sulfide which combines with iron present in the medium forming black precipitate.
Some bacteria such as Bacillus sps can hydrolyze starch (amylose and amylopectin) using the enzymes α-amylase and oligo-1,6-glucosidase.
Furazolidone disk test is used to differentiate staphylococci (sensitive) from micrococci (resistant).
Kligler Iron Agar is used for the differentiation of microorganisms on the basis of glucose and lactose fermentation and H₂S production.
It tests for the presence of acid and/or gas produced from carbohydrate fermentation. and is useful in differentiating among bacterial groups or species.
The most widely used anaerobic culture medium containing chopped meat particles and used for maintenance of stock cultures.
Some organism such as Proteus oxidatively deaminate phenylalanine to phenyl-pyruvic acid.
The oxidative-fermentative (OF) test is used to differentiate oxidative bacteria from fermentative bacteria based on acid production.
Elek test is an in vitro immunoprecipitation test to determine whether or not a strain of Corynebacterium diphtheriae is toxigenic.
These tests determine if an organism is motile (possesses flagella).
Lecithinase test is a biochemical test used to identify organisms that liberate phospholipases (lecithinases) e.g. Clostridium perfringens.
Acetate utilization test determines if an organism can use acetate as the sole source of carbon.
Rapid test for identifying Moraxella (Branhamella) catarrhalis which produce butyrate esterase enzyme.
Cetrimide agar test the ability of an organism to grow in the presence of cetrimide.
API-20E test strip (from bioMerieux, Inc.) is used to identify the enteric gram negative rods.
The nitrocefin test is a sensitive technique for detecting beta-lactamase-producing strains of N. gonorrhoeae, H. influenzae, Enterococcus spp.
Certain bacteria produce acetoin as the chief end product of glucose fermentation. Acetoin gets oxidized to diacetyl giving red color.
The presence of lysine, ornithine, and arginine decarboxylase enzyme is routinely tested in the identification of Enterobacteriaceae.
Hippurate test is used in the identification of Campylobacter jejuni, Listeria monocytogenes, Gardnerella vaginalis, and Streptococcus agalactiae.
Beta hemolytic streptococci are arranged into twenty (20) groups A-U without I and J (i.e. A through H and K through V).
Mycobacterium species possess an enzyme-lipase, that splits Tween-80 and releases oleic acid and polyoxyethylated sorbitol.
Organism producing β-galactosidase hydrolyzes the ONPG producing visible yellow product (orthonitrophenol).
Nitrate reduction test is used to determine the ability of an organism to reduce nitrate.
Common culture media are bismuth sulfite agar, MacConkey agar, deoxycholate citrate agar, xylose lysine deoxycholate agar, salmonella-shigella agar, etc.
V. cholerae emulsified in sodium deoxycholate, it lyses the cell wall and releases DNA giving a mucoid “string” when a loop is drawn away.
DNase test determines the ability of an organism to hydrolyze DNA.
Certain bacteria ferment glucose to produce stronger acids that take the media pH below 4.4 in which Methyl Red indicator turns red.
Gelatin hydrolysis test determines the ability of an organism to produce gelatinases that liquefy gelatin.
Bile-esculin test is widely used to differentiate Enterococci and Non-enterococcus group D streptococci from non-group D viridans group streptococci.
Salt tolerance test determines the ability of an organism to grow in high concentrations of salt (6.5%).
Some gram-positive cocci such as Enterococcus produces enzyme L-pyrroglutamylaminopeptidase that hydrolyze PYR substrate.
Group B Streptococci produce CAMP factor that acts synergistically with beta-lysin of S. aureus.
Catalase test is used to distinguish among Gram-positive cocci. Staphylococci are catalase-positive but Streptococci are catalase-negative.
Weil-Felix is a nonspecific agglutination test that detects anti-rickettsial antibodies in patient serum using OX strains of Proteus vulgaris.
Tests are; Tween-80 hydrolysis test, niacin test, nitrate reduction test, tellurite reduction test, urea hydrolysis test and citrate utilization test.
Novobiocin test is used to differentiate CONS and presumptively identify the isolate as Staphylococcus saprophyticus (novobiocin resistant).
IMViC is an acronym that stands for four different tests; indole test, methyl red test, Voges-Proskauer test, and citrate utilization test.
Escherichia coli produces the enzyme beta-D-glucuronidase, which hydrolyzes a substrate giving blue fluorescence.
TSI test determines whether a gram-negative rod utilizes glucose, and lactose or sucrose fermentatively and forms hydrogen sulfide.
Optochin (ethylhydrocupreine hydrochloride) lyses pneumococci (positive) but alpha-streptococci are resistant (negative).
Bile solubility test differentiates S. pneumoniae (positive) from alpha-hemolytic streptococci (negative).
Citrate test determines the ability of an organism to utilize sodium citrate as source source of carbon.
Bacitracin test differentiates S. pyogenes (inhibited) from other beta-hemolytic streptococci.
Catalase test, oxidase test, MUG test, optochin sensitivity test, bacitracin sensitivity test, coagulase test, etc are some of the common biochemical tests.
Characteristics pigmentation observed in the culture media is because of the changes in the pH of the medium.
This test determines the ability of an organism to produce the enzyme urease which hydrolyzes urea.
Organism having cytochrome oxidase oxidizes tetramethyl-p-phenylenediamine hydrochloride to indophenol (a dark purple-colored) compound.
Coagulase test detects clumping factors and bound coagulase and differentiates Staphylococcus aureus (positive) from CoNS (negative).