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Lab 5
| Question | Answer |
|---|---|
| What is tubridity in nutrient? | Cloudness in liquid media that indicates microbial growth |
| How do we know if there is growth? | There will be colonies on the slant |
| How might we be able to tell if there is contamination? | If there are distinctly different colonies present |
| What is the purpose of selective media? | To either allow for or pervent a specfic groups of microbes from growing in the medium |
| What is the purpose of differential media? | To cause the growth of microbes to appear different, usually based on some dfference in metabolism |
| What makes MSA selective? | High salt-concentration prevents frowth of microbes that are not halo-tolerant |
| What types of micrboes can grow on MSA? | Halo-toleratn microbes, such as Staphylococcus species |
| What makes MSA differential? | Fermentation of mannitol produces acid that will lower the pH of the medium, causing the medium to turn yellow |
| How does growth look different for Staphylococcus aureus compared to Staphylococcus epidermidis? | Staphylococcus aureus ferments mannitol, so the medium around it will turn yellow. Staphylococcus epidermidis can tolerate salt but does not ferment mannitol, so it’s growth will be colorless with no color change in the medium |
| What does colorless growth on MSA tell you about the bacteria? | The bacteria can tolerate salt but does not ferment mannitol – like with Staphylococcus epidermidis |
| What does a yellow halo around your microbe tell you about the bacteria? | The microbe ferments mannitol (and tolerates salt) – like with Staphylococcus aureus |
| What 2 things make MacConkey agar selective? | Bile salts (prevent growth of non-enteric microbes) and crystal violet (prevents growth of Gram positive bacteria) |
| What types of microbes can grow on MacConkey agar? | Enteric Gram negative bacteria |
| What makes MacConkey agar differential? | Lactose fermentation causes magenta-colored growth |
| What does colorless growth on MacConkey agar tell you about the bacteria? | Bacteria is enteric and Gram negative but does not use lactose – like Salmonella typhi |
| What does magenta growth on MacConkey agar tell you about the bacteria? | Bacteria ferments lactose (and is enteric and Gram negative) – like E. col |
| How does growth look different for E. coli compared to Salmonella typhi? Why | Shigella flexneri will have colorless growth due to no lactose use and no sulfur reduction. Salmonella typhi will have colorless growth with a black precipitate due to sulfur reduction and no lactose use. |
| Should Staphylococcus aureus grow on Hektoen-Enteric agar? Why or why not? | No; Staphylococcus aureus is not enteric, so it’s growth should be inhibited. |
| What does it mean if a bubble is present in the Durham tube? | Gas was produced from carbohydrate use |
| What does a positive result for carbohydrate fermentation look like? Why? | Medium turns yellow due to acid produced from carbohydrate use |
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