i am married in the uk and have had a one night stand with a work colleague which was unprotected and involved multiple incidents of intercourse over a night (around ten). 

This partner is in a long term relationship but was seeing someone outside of this relationship 18 months ago. She tested for ghonnoreah and chlamaydia at the end of january and was negative.

i am plan to to test for ghonnoreah and chlamydia for peace of mind but am confused as to the need to test for M Genitalium and Trich (i can test for both privately). From everything i’ve read on your site it seems that M. Gen testing is only recommended if symptoms are experienced. I don’t understand this, if this is an infecrion that is known to be often asymptomatic why is routine testing not recommended?

I also don’t understand the risks of trich - my wife and i are in our early 40s and do not plan to have children. Is there any real risk to my wife’s health ? if trich is very common but with little risk to health outside of pregnancy complications, i don’t want to run the risk of testing positive and have to tell my wife and risk my marriage for neglible health risk to my wife.

If you were me, what would you be tested for? 

Let me start with a general comment,  Odds are that your casual partner does not have an STI- you don't mention when the encounter occurred but I presume it was around the time of her negative tests.  If that is thecase, unless there is some other reason for concern, it's unlikely she was infected;

Nonetheless I understand your desire to be cautious and be tested.  Be sure to test for gonorrhea and chlamydia at all sites of exposure.  Testing for trichomonas would be appropriate for the same reason that you want to test for gonorrhea.  Trich is actually as or sometimes more common that gonorrhea or chlamydia in some populations and asymptomatic infections (in your casual partner or, if transmitted, you) can then be passed on to partners to cause symptomatic infections.  There do not seem to be complications, other than the potential discomfort of symptoms associated with trich as there are in gonorrhea or chlamydia.

Mycoplasma genitalium is more complex.  Recommendations vary depending on location.  In the United States and according to the WHO, testing for M. gent should be deferred unless patients have persistent symptoms after being tested and treated for for gonorrhea and chlamydia.  The reason for this is that the  natural history of M. gent is poorly understood and while the infection can be present in symptomatic persons, infections also appear to go away without symptoms in some patients and the direct evidence that M. gent causes complications is lacking (there are associations, but not proof- it's complicated).  OTOH, in the UK, testing and efforts to treat M. gent is more common, experts in the UK having decided that the evidence is strong enough to act upon.  

If I were you, I would not seek testing for M. gent but would test for trich as well as gonorrhea and chlamydia, if you even choose to test (I am not sure I would).  I would like to see more evidence.  Admittedly however, there are other experts who would suggest testing to be thorough.

I hope that I've succeeded in providing you with both sides of this controversial issue (M. gent testing)EWH