The genome of most prokaryotic organisms consists of double-stranded DNA organized into one circular chromosome in a region of cytoplasm called the nucleoid. The chromosome is tightly wound, or supercoiled, for efficient storage. Prokaryotes also contain other circular pieces of DNA called plasmids. These plasmids are smaller than the chromosome and often carry genes that confer adaptive functions, such as antibiotic resistance.
Although bacterial genomes are much smaller than eukaryotic genomes, they vary considerably in size and gene content. One of the smallest known bacterial genomes is that of Mycoplasma genitalium, a sexually transmitted pathogen that causes urinary and genital tract infections in humans. The M. genitalium genome is 580,076 base pairs long and consists of 559 (476 coding and 83 noncoding) genes. On the other end of the spectrum lies a particular strain of Sorangium cellulosum, a soil-dwelling bacterium. The S. cellulosum genome is enormous for a bacterium at 14,782,125 base pairs long, encoding 11,599 genes.
Before the discovery of antibiotics, minor injuries could turn deadly due to the inability to stop simple bacterial infections. The discovery of penicillin in 1928 ushered in the antibiotic era, characterized by revolutionizing medical treatments and an increase in life expectancy. However, the overuse of antibiotics in humans and agricultural animals has caused some bacteria to evolve resistance to antibiotics, rendering them less effective or ineffective. Antibiotic resistance genes can be carried on plasmids, which is problematic because many bacteria can exchange plasmids with distantly related species through a process called bacterial conjugation. Antibiotic resistance can, therefore, spread quickly through bacterial populations, highlighting the urgent need to develop new antibiotics.
