Identifying an unknown - S. flexneri and P. vulgaris with lactose fermentation? - (Apr/25/2010 )

I was able to narrow down my unknown to either Proteus Vulgaris or Shigella flexneri however the lactose test seems to contradict this deduction and other test contradict narrowing it down further. Any help on making the final decision or explaining erroneous results is much appreciated. I am not able to perform anymore test.


1) The unknown tested positive (A/G) for lactose fermentation (both MOs should read negative).
2) The unknown tested negative for hydrogen sulfide (true for S. flexneri only).
3) The unknown tested positive for lipase (true for P. vulgaris only).
4) The unknown did not grow on MSA (true for P. vulgaris only).

The growth on the EMB plate was a purple color/not very iridescent which indicates a non-lactose fermentation, correct?. If so, then why would I get a positive lactose test for reasons other than contaminated sample or different strains?

For your edification the other test results which lead me to these two choices out of 30 possible MOs include:
gram negative,
bacillus,
light gray on TSA,
growth at 37 degrees,
oxidase (+),
catalase (-),
sucrose (A/-),
glucose (A/G),
indole (+),
MR (+),
VP (+),
VP (-),
Citrate (-).

I am leaning towards P. vulgaris but I am not sure how to justify my final decision given the contradiction in the test. Any thoughts on my choice or how to justify this? Thanks for any help.

If I am not mistaken then a purple (or green) color means its positive on an EMB plate... not negative like you say.


I have serious doubts on your pick of bacterium.

PS. what do you mean with bacillus? Bacillus is gram positive...

Thanks for the response, Pito.

pito on Apr 25 2010, 10:39 AM said:

I'm starting to doubt my deduction too. I'm going to look again at the list of possible choices. Thanks.

mpoper on Apr 25 2010, 12:51 PM said:

I am thinking that neither Shigella nor Proteus are lactose fermenters and the fact that the EMB is growing prurple colonies demonstrates lactose fermentation. Also neither Shigella nor Proteus are oxidase positive. Plus, usually if the MR is positive the VP will be negative. Is it possible that you have a mixed culture?

Good luck.