LP16 Food, Numbers and Selective Media

(1) Multiple methods exist for determining the number of organisms present in a broth culture.

1. Direct microscopic counts involve slides with grids, counting cells per square and multiplying times the total number of squares.

(2) Multiple methods exist for determining the number of organisms present in a broth culture.

2. Turbidimetric methods involve instruments sensitive to changes in turbidity, e.g., this spectrophotometer.

(3) Multiple methods exist for determining the number of organisms present in a broth culture.

3. Electric counting devices such as this Coulter counter count cells one at a time as they pass through an opening.

What fermentation product is causing the color change in this BCP lactose agar?

Fermentation of lactose causes acidification of the broth, which causes the pH indicator BCP to change from purple to yellow

What bacteria genera are most commonly found in yogurt? What fermentation products give yogurt its distinctive flavor?

Lactobacillus d. bulgarians & Streptococcus thermophiles; lactic acid & acetaldehyde

What organisms are doing the fermentation here, and what are their products?

Saccharomyces cerevisiae; ethanol & carbon dioxide

Do you remember the health benefits of eating fresh yogurt? Do you remember learning about some food-borne pathogens?

The active cultures act as probiotics for your colon (gut); Salmonella enterica, Escherichia coli, Vibrio. Intoxication: Clostridium botulinum, and Staphylococcus aureus

None of these methods can be used to determine the number of living cells in a culture; a factor significantly influencing microbial growth.

To determine the number of living cells present in a broth culture, we can use the Quantitative Plating Method or Viable Cell Count.

Which cultured food, the yogurt or the buttermilk contains organisms that produce more acid? Why?

Yogurt is essentially cultured so that the probiotics convert lactose to lactic acid, increasing the acidity of the final product

replica plating

a transfer technique that may be used to produce multiple copies (replicas) of a particular colonial arrangement on a number of agar plates.

Differential media

are those that allow for the growth of multiple different types of microorganisms while causing them to appear different. For example, the bromocresol purple lactose agar (BCP lactose) is a differential medium

Selective media

are those that promote the growth of certain types of micro-organisms while inhibiting the growth of others. For example, Pseudomonas and Azotobacter enrichment media

What is this fermented food called and what organisms are responsible for the fermentation process?

cabbage; sauerkraut - Leuconostoc mesenteroides, Lactobacillus brevis & plantarum

Mixing milk, buttermilk and rennet, allowing the mixture to sit in a warm location for about an hour will form?Which is the curd, and which is the whey?

cheese; the solid, curdled lumps are the curds, the whey is the liquid byproduct

complex media

contain nutrients in crude form, i.e., their exact type and quantity is not known.

defined media

contain nutrients in pure chemical form and the exact quantity of each one is specified.

What is the name of this material and what organisms are responsible for the fermentation? What fermentation product is exiting through the rubber tubing? What product stays in the flask?

fermentative organisms: saccaromyces cerevisiae; product exited: carbon dioxide; product(s) stayed: yeast (and ethanol)

Which of these materials are fermented foods, and which are cultured foods? How do you know?

fermented: sauerkraut cultured: cheese, apple wine Fermented foods only contain the product(s) of fermentation (e.g. lactic acid) while cultured foods not only contain fermentation products but also live microbial cultures

What bacteria genera are most commonly used in the production of cheese?

genus: Lactococcus species: lactis

What change in pH is evident here, and what is the name of the fermentation product causing the change?

pH change: from neutral to acidic; fermentation product: lactic acid bacteria

Are these cheeses ripened or unripened? How do you know?

ripened; to touch the cheese to check if it's hard with low water content

serial dilution

the process of diluting a bacterial culture stepwise through a series of tubes containing known amounts of solution

Soft cheeses differ from hard cheeses in that they contain more of what substance?

water