Submitted on: 30 June 2013.
Revised on: 14 September 2013.
Accepted on: 27 October 2013.
___________________AIMS Microbiology---AMER INST MATHEMATICAL SCIENCES-AIMS
Submitted on: 22 June 1999.
Revised on: 04 October 1999.
Accepted on: 30 October 1999.
___________________AIMS Microbiology---AMER INST MATHEMATICAL SCIENCES-AIMS
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