3 Quick checks to ensure believable micro results
How do you know if your micro results make sense?
November 20, 2017
Sometimes the amount of data your microbiology lab sends you ends up hiding the information. So many not detected or “<” (less thans) it can be hard to be sure all is correct. We also know those pesky bacteria don’t always follow hard and fast rules like chemical compounds are more likely to do – so when do you feel justified to challenge your lab on their micro results?
Here are a few red flags to watch out for:
Quick Check #1
TVC result should be higher than your Coliforms, Enterobacteriaceas (Enteros) and E coli results
TVC (total viable count, also known as ACC or APC (see a previous blog post) is the number of bacteria we can get to multiply under aerobic (with oxygen) conditions, at 30C within 48 (or 72) hours. There may be other bacteria there which require other growth conditions but this is the method for TVC counts in food. So if in theory all the aerobic bacteria should be detected on the TVC plate, therefore you should not find Enterobacteriaceae, Coliforms, Staphylococcus species, Pseudomonads, E coli or other aerobes (bacteria capable of growing in the presence of oxygen) counts being higher than your TVC result.
Quick Check #2
Coliform and Enterobacteriaceae results should be pretty much the same and both should have higher results than E coli
All Coliforms are Enteros (Enterobacteriaceae) but not all Enteros are Coliforms. Most people don’t test for both at the same time because they are almost interchangeable; but should you get a significantly higher result for Coliforms over Enteros something is not right (I would accept up to 100 Coliforms to < 10 Enteros, but not 1000 Coliforms and 100 Enteros!). Another thing – E coli are Coliforms and thus also part of the Enterobacteriaceae group; and since they are just one species within this very large group it is very rare for a lab to find E coli and no Coliforms or Enteros! *
Quick Check #3
Where there are a lot of Lactics and Pseudomonads there should be a lot of TVCs 3.
This is a little bit harder to explain and not as black and white as Quick Checks 1 and 2, but still one to watch out for. Since Lactic acid bacteria can grow aerobically and Pseudomonads are strict aerobes they are capable of growing on a TVC test (see quick check fact #1). So if there are a lot of lactics – you should have a lot of TVC and the same for Pseudomonads. The only thing is there may be lots of other aerobic bacteria so the TVC result could be much higher and because the agar used to grow the Lactics and Pseudomonads is specially prepared for these bacteria they can sometimes be better at recovering these bacteria than the non-speciifc agar used in the TVC test. But again if you see a > 1000 Pseudomonads and no TVC….. It may be worth requesting a retest.
Now for a few things you cannot assume:
1. You cannot assume that if there are no Enterobacteriaceae there will not be any Salmonella, even though Salmonella are also Enteros. The tests for Salmonella are much more sensitive than that for Enteros – so although the chance that Salmonella is present goes up the more Enteros you have, having none doesn’t rule the risk out.
2. Having no E coli means there is no E coli O157. E coli are indicator organisms which won’t make anyone sick but E coli O157 is one strain which does make people very sick. Testing only for E coli cannot rule out the absence of E coli O157 for two reasons: (1) the same as in the case of Salmonella testing for E coli O157 is much more sensitive than testing for E coli in general. (2) E coli O157 actually isn’t detectable on the routine E coli test! For more information on this watch out for our blog on E coli O157 coming soon.
And one final word of warning: Microbiology deals with living things so they don’t always follow hard and fast rules, but these few simple checks should give you confidence your lab is doing things right.
* It is a different test and due to variation on the limits of uncertainty it can happen that 10 E coli are reported and < 10 Enteros on the same sample. But this should be the exception not the norm.