Should bacteriostatic cream sustain bacterial survival for any length of time? - (Mar/31/2016 )

I am a high school science instructor, and am trying to understand some results from a bacterial culture experiment which we did. One thing we examined was whether hand creams might harbor bacteria. One tube of cream which we tested, a brand which states that it has bacteriostatic properties (does not mention type of bacteriostatic agent), clearly showed evidence of bacterial cultures. (Colonies appeared clearly in 3 separate agar dishes, while no colonies appeared in controls). We compared both a used tube - which did show clear colonies, to a new (previously unopened) tube - which showed NO colonies (in all 3 agar dishes). It appears clear that the tube became contaminated after prior use.

The questions we have (which I could not answer) were:

1) Does it make sense that a cream with supposed bacteriostatic properties can harbor bacteria? (I know that bacteriostatic does not mean that it kills bacteria - only stops growth - but could bacteria in fact survive for any length of time in such an enviroment? The tube in which bacteria were found had not been used - and was tightly closed - for over a month. So if it did become contaminated, the bacteria would have to survive there for a long time...)

2) Or is it more likely that the cream simply does not have bacteriostatic properties, and bacteria were in fact able to reproduce and colonize the cream after initial contamination (in which case, we might want to contact the company and see what they claim...)

-BioTeacher-

BioTeacher on Fri Apr 1 04:02:34 2016 said:

I am a high school science instructor, and am trying to understand some results from a bacterial culture experiment which we did. One thing we examined was whether hand creams might harbor bacteria. One tube of cream which we tested, a brand which states that it has bacteriostatic properties (does not mention type of bacteriostatic agent), clearly showed evidence of bacterial cultures. (Colonies appeared clearly in 3 separate agar dishes, while no colonies appeared in controls). We compared both a used tube - which did show clear colonies, to a new (previously unopened) tube - which showed NO colonies (in all 3 agar dishes). It appears clear that the tube became contaminated after prior use.

The questions we have (which I could not answer) were:

1) Does it make sense that a cream with supposed bacteriostatic properties can harbor bacteria? (I know that bacteriostatic does not mean that it kills bacteria - only stops growth - but could bacteria in fact survive for any length of time in such an enviroment? The tube in which bacteria were found had not been used - and was tightly closed - for over a month. So if it did become contaminated, the bacteria would have to survive there for a long time...)

2) Or is it more likely that the cream simply does not have bacteriostatic properties, and bacteria were in fact able to reproduce and colonize the cream after initial contamination (in which case, we might want to contact the company and see what they claim...)

For sure, bacteria will survive in those creams.

The bacteriostatic effects: not sure what kind of cream it is, but I am guessing its not some sort of professional disinfecting cream?

But I am a bit confused: first you say that the not opened tube was free of contamination and then later on you state that you found them in a tube that was not been used?

The bacteria do not need to be "alive" while in the cream. It might be bacterial spores that just survive.

Or persister cells...

Hard to tell, but its not unlikely that bacteria from your hand get into the cream ... this is something known.

And the cream can be bacteriostatic, but as you already said: it does not kill them.

-pito-

What titer do you estimate in the product? If it is a "bacteriostatic cream", its active may not include the contaminat you found. For examples, some Gram negative bacteria are not affected by quats and triclosan and the viable units you're recovering may represent bacterial or fungal spores.

Are you neutralizing the active ingredient in culture?

-Phil Geis-