How to handle carefully back-biting colleagues who happens to be your acquaintance?

This behaviour results from passive (negative) leadership. A teamleader should be positive and empower employees. In your case the only thing you can do just be better than the others in work performance, discipline and attendance. The day will come that all your positive actions will change the attitude of your colleagues or you will be promoted. Good luck in your career.

Once you come to know they are back biting keep them away or else let them know you learnt what they do, as you can give a clear chance for them to change their behaviour or else keep them away who wants to hurt you as they cant be called "Aquaintances"

first of all how can you have as an a quaintances that you know are back-biting.,the reason they are biting is that they know you too well and know you will not do anything about it. would you ever feed your hand inside a lions mouth even  if it he or it was your pet???

loose them it is better, energy drainers and back-biters will always be what they are they will hold  you back,hurt you and put you in a negative mood soon.

After many Years experience in Family, Neighbourhood, Class Rooms & then in Job; I have learnt only two things:

1. Person who doesnot Socially Inter-act & Inter-connect, is Back-Bited most. So be part of your related Persons Gatherings.
2. Also Give Individual Time to each of them so that they may vocal you positvely when you are not in any Gathering.

From my point of view, and as a Muslim man, I will behave as our Prophet Mohammad (PBUH)  directed us to do: simply perform what ever kind tasks you are asked to do to your relationship peoble as best as you can, and ask God (Allah) whatever those guys (backbiting acquaintance) should do for you (in addition to avoiding and ignoring their immoral actions against you). 

By doing this for some time, backbiting acquaintance will eventually not only stop hurting you, but may be changed to the other side by doing kind things for you (hopefully!).