It’s the affinity issue – ATM has a high affinity to penicillin-binding protein 3 (PBP3) of Gram-negative bacteria, but doesn't bind well to PBP's of aerobic Gram-positives.

@Oliver Nolte Thankyou sir for the confirmation but do you have any reference where a weaker binding towards Gram-positive bacteria reported? It will be really very useful.

Thanks once again.

Try to get this article:

Aztreonam. A review of its antibacterial activity, pharmacokinetic properties and therapeutic use.  by: Brogden RN and Heel RC.

Thank you Alaa Abed, I got that review.