Question

If you inoculated an unknown bacteria culture onto PEA agar, anr it grew, is it more likely that the unknown bacteria are gram-positive or gram-negative? why? What can uoi conclude aboit E. coli because it did not grow on MSA?

Answer 1

PEA agar stands for Phenylethyl Alcohol Agar. Any selective medium that contains PEA agar will allow growth of gram positive bacteria only.

It interferes with the DNA synthesis of Gram negative organisms and hence inhibits their growth. Along with gram negative bacteria, PEA agar also inhibits growth of fungi and Proteus species.

Thus, the unknown bacterium in this question is gram positive.

MSA has high salt concentration (which act as a selective ingredient) to allow growth of Staphylococcus species and inhibit the growth of Streptococcus species.

Thus, it can be concluded that growth of E. coli is inhibited by the high salt concentration.