Physiological Grouping of Bacteria

Physiological Grouping

Acid producers

The organisms capable of fermenting lactose to form lactic acid are known as acid producers. Lactic acid coagulates milk by producing precipitation of Casein at 4.6 pH.

Homofermenters: Lactococcus, few Lactobacilli

Heterofermenters: Few Lactobacilli, Lueconostoc spp.,

LAB are classified as heterotrophic chemoorganotrophs i.e. these require the preformed carbon and energy sources. LAB lack cytochrome and cannot use respiratory pathway for energy generation. LAB generally lack metabolic diversity and mainly relies on two types of fermentation for the metabolism of sugars that form the basis of their taxonomic characterization. Based on Embden Meyerhoff Pathway and Phosphoketolase pathways LAB are classified as follows:

·         Homofermentative LAB: These Homofermentative LAB group use Embden Meyerhoff Pathway for hexose sugars utilization and produce 2 pyruvate and 2 ATP molecules. Pyruvate is further converted to lactic acid by the enzyme lactate dehydrogenase and is the only end product. However, under certain conditions the homofermenters can also undergo heterolactic fermentation. For example: during pentose sugar fermentation by LAB.

·         Heterofermentative LAB: This group of bacteria catabolise hexose sugars by Phosphoketolase Pathway and produce an equal amount of lactic Acid, carbon dioxide, acetate and ethanol as the major end products.

·         During this process only one ATP molecule is produced.

Gas producers

Gas producers are the organisms capable of producing CO₂ or \ and H₂ from lactose fermentations (e.g. E. coli, yeasts, and Clostridium spp.).

Proteolytic

Proteolytic organisms degrade milk proteins into soluble components by enzymes known as proteinases or proteases (e.g. Bacillus spp, Pseudomonas spp.).

Lipolytic

Lipolytic organisms are capable of attacking milk fat by enzymes such as lipases liberating glycerides and fatty acids (e.g. Pseudomonas spp, Achromobacter lipolyticum.

Moulds: Geotrichum candidum, Penicillium roqueforti).

Sweet curdling

Organisms capable of causing curdling of milk by rennin like enzyme before the development of sufficient acidity are known as sweet curdling organisms (e.g. B. subtilis, B. Cereus and Enterococcus liquefaciens).

Ropiness

Ropiness organism produce change in the viscosity of milk or forming threads when the milk is poured from one container to other, due to production of gums, mucins etc. (e.g. Alcaligenes viscosus).

Flavour producing

Many organisms are capable of producing different flavours in milk during their growth. Some of the flavours and associated organisms are listed below:

Fruity Pseudomonas fragi

Malty Lactococcus lactis subsp. lactis biovar maltigenes

Fishy Proteus icthyosmius

Unclean E. coli

Colour fermentations

Many organisms are capable of producing different colours in milk during their growth by producing chromogenic compounds. Some of the colours and associated organisms are listed below:

Yellow:          Pseudomonas synxantha

Blue:              Pseudomonas cyanogenes

Green:            Penicillium roqueforti

Black:             Pseudomonas nigrifaciens