Microbiology Lab Techniques: Lactose Fermentation and Aerobic Pathways Quiz

Nonpathogenic Lactose Fermenters are bacteria that can ferment lactose but are not known to cause significant illness in humans. E. coli, Enterobacter, and Klebsiella are examples of nonpathogenic lactose fermenters, while Salmonella, Staphylococcus, and Streptococcus are not part of this group.

Proteus is a nonpathogenic lactose non-fermenter often used as an example in microbiology for this category of bacteria.

Pathogenic Lactose Non-fermenters are bacteria that do not ferment lactose and are capable of causing infections. Salmonella and Shigella are both examples of such bacteria, whereas Escherichia coli, Klebsiella pneumoniae, and Enterococcus faecalis are not typically classified as pathogenic lactose non-fermenters.

Cytochrome c oxidase picks up electrons and passes them to a pigment called cytochrome c, which is part of the electron transport pigment system needed for aerobic respiration. This oxidase reagent acts as a false electron donor so if Cytochrome c oxidase is present, it will take the electrons from the oxidase reagent. When the reagent becomes oxidized, by giving away the electrons, it turns purple.

Phenol red lactose broth is a differential media used to distinguish between lactose fermenters and non-fermenters based on color change. Escherichia coli typically shows a positive result with a yellow color due to acidic byproducts of lactose fermentation and gas production. On the other hand, Salmonella typhimurium usually shows a negative result with an orange/red color indicating a lack of lactose fermentation and gas production.

Phenol red is a pH indicator commonly used for detecting the pH of solutions. It turns cerise pink under alkaline pH conditions, red when lactose is present at neutral pH, and yellow in acidic (positive) pH conditions.

MAC agar is selective for gram-negative bacteria that can ferment lactose and are bile salt/dye tolerant, while inhibiting the growth of gram-positive bacteria and non-pathogenic bacteria.

On MAC agar, lactose fermenters produce pink or pinkish-colored colonies due to the acid byproducts of lactose fermentation. This color change is a characteristic feature used to differentiate lactose fermenters from other types of bacteria.

MacConkey's Agar (MAC) is a selective and differential agar used to isolate and differentiate lactose fermenters from nonfermenters. Clear or colorless growth on MAC indicates that the organism is unable to ferment lactose, leading to the classification as a Lactose nonfermenter (LNF). The incorrect answers do not match the observations on MAC agar.

On EMB agar, only organisms that are able to withstand the dyes present in the media will be able to grow, indicating dye tolerance.

EMB agar is selective for Gram-negative bacteria and can differentiate between lactose fermenters and non-lactose fermenters based on the color of growth. Dark purple or purplish growth indicates lactose fermentation.

EMB agar differentiates between lactose fermenters and nonfermenters based on the color of colonies. Lactose nonfermenters appear transparent on EMB agar.

Hektoen Enteric Agar (HEA) is selective for Gram-negative enteric pathogens as it contains bile salts and dyes that inhibit the growth of Gram-positive organisms. Anything that grows on HEA indicates bile salt/dye tolerance.

The bright orange to salmon pink color on HEA indicates the utilization of lactose as a carbon source, which is a characteristic of lactose fermenting organisms.

HEA is a selective and differential agar used to isolate Salmonella and Shigella species while inhibiting the growth of gram-negative bacteria. Lactose nonfermenters appear green/clear/colorless on HEA.

HEA is a selective and differential agar that not only differentiates based on lactose fermentation but also on hydrogen sulfide production. Green growth with black precipitation indicates a lactose nonfermenter that produces hydrogen sulfide.

MAC media characteristics: selective and differential. Look for signs of growth vs. no growth from the selective components of the MAC (bile salts and crystal violet). Bile salts and a small amount of crystal violet can inhibit the growth of gram-positive bacteria. Also look at the color of any colonies present due to the differential components of MAC (lactose and neutral red pH indicator). Lactose fermenters will be pinkish due to the uptake and fermentation of lactose and neutral red, while Lactose nonfermenters will be colorless due to the lack of uptake of lactose and neutral red.

EMB media characteristics: selective and differential. Look for signs of growth vs. no growth from the selective components of EMB (eosin-y and methylene blue). Eosin-y and methylene blue can inhibit the growth of gram-pisitive bacteria. Also look at the color of any colonies present due to the differential components of EMB (lactose, eosin-y and methylene blue). These inhibiting agents combine to form a precipitate at acid pH thus serving as indicators of acid production. Strong lactose fermenters will be dark purple/black with a metallic sheen due to the update and fermentation of lactose, slow or weak lactose fermenters will be purple, and lactose nonfermenters will be transparent due to the lack of uptake of lactose.

HEA media characteristics: selective and differential. Look for signs of growth vs. no growth from the selective components of HEA (bile salts). Bile salts can inhibit the growth of gram-positive bacteria. Also look at the color of any colonies present due to the differential components of HEA (lactose, acid fuchsin, bromthymol blue pH indicator, and ferric ammonium citrate). Lactose fermenters will be bright orange to salmon pink due to the uptake and fermentation of lactose. Bromthymol blue and acid fuchsin dyes are added as color indicators. Ferric ammonium citrate is added to react with H2S and forms a black precipitate (dots). Lactose nonfermenters and non-Hydrogen sulfide producers will be green due to the lack of uptake of lactose. Lactose nonfermenters and H2S producers will be green with black precipitate.