I would NOT recommend taking any lab science course online. Before enrolling in an online science course, I would ask the programs I am interested in applying to if they will accept the course.
I currently live in the NY metro area and took my prerequisite course work at a community college. The community colleges offer these courses in the evenings and on weekends to accommodate students who are working full time during the working week. I took Anatomy and Physiology I on the weekend Spring 2011 which consisted of a three hour lecture on Saturday and three hour lab on Sunday. My weekend was devoured by A &P I.
I know that Microbiology can be a particularly difficult class to enroll in due to space constraints. If you are persistent, you will be able to get a seat.
I also would not recommend taking more than one lab class and working full time unless you are in a mad rush to finish prerequisite and don't mind punishing yourself.
Community colleges are the way to go for prerequisite course work. The schedules are convenient and the classes are inexpensive compared to private schools. I saved my money for the actual nursing program rather than the generic prerequisites.
Finally, most programs permit submission of application prior to completion of all prerequisite courses provided all courses are finished prior to matriculation in the program. The more prerequisites completed at the time of application with A's, especially the sciences, the more competitive your application.
Any more questions, please ask! I just completed my application and acceptance process.