I am providing a link here to an excellent review of gut bacteria published by the National Institutes of Health--check it out--its good.  It has great pictures and accurate information regarding the dangers of antibiotics.  Here are a few quotes:

The microbiota is similar to an organ in that it performs functions essential for our survival. And just as with the heart or the lungs, when an environmental agent alters the function of the microbiota, the result can be disease.

Both human and animal studies have shown that even a one-time antibiotic treatment can lead to long-term shifts in microbial populations. The health consequences of these long-term shifts are still largely unknown.

“The incidence of [H. pylori] infection has been declining in the United States and other developed countries, which may contribute to the increasing incidence of [esophageal adenocarcinoma].”

In a study reported by Johan Dicksved et al. in the April 2007 issue of Applied and Environmental Microbiology, children raised according to the “anthroposophic” lifestyle touted by philosopher Rudolf Steiner—with restricted antibiotics and plenty of microbe-rich fermented foods—showed higher microbial diversity than farm children, whose diets included more farm-produced animal products.

Psychological stress appears to reduce the numbers of Lactobacilli species in the human gut while increasing the growth of pathogens such as Escherichia coli and Pseudomonas species, as reported by Femke Lutgendorff et al. in the June 2008 issue of Current Molecular Medicine. 

"Helena Parracho and colleagues reported in the October 2005 Journal of Medical Microbiology that 91.4% of 58 autistic children studied had a GI disorder, compared with 25% of otherwise-healthy siblings..."

Several studies have reported a strong correlation between disrupted microbial composition and allergies and asthma. Infants with atopic eczema had lower microbial diversity in their guts as well as fewer species of Bifidobacteria, compared with healthy infants, according to a study by Mei Wang et al. in the January 2008 Journal of Allergy and Clinical Immunology.

The bacteria in a gut, to a larger extent than I think we appreciate, both reflect and determine the state of your health.  Antibiotics are incredibly over-prescribed to the detriment of the user.