What common features are found in bacterial genomes that have been sequenced?
The genome of E. coli (sequenced in 1997) is about 4 million base pairs with about 3000 genes. These numbers are quite average for bacteria; i.e., most have a genome size of several million base pairs containing a few thousand genes. (For example, in a study done here at Lehigh in the 1990’s, we found that the pathogenic bacterium Clostridium difficile has a genome size of 4.4 million base pairs ( Norwood and Sands, Gene vol. 201, 159-168 (1997) ). Thus, bacterial genomes are only about 0.1% as big as the human genome, and have about 10% as many genes as we do. A comparison of those two percentages shows immediately that in bacteria the “gene density” (how many genes there are per unit length along the genome) is much higher than in humans. That is, whereas a one million base pair length in us contains on average about 10 genes, one million base pairs of bacterial DNA contains about 500 to 1000 genes. This much greater gene density is due to a combination of factors: (1) bacterial gene
