Bacterial genetic material is organized as a single circular chromosome compacted in the nucleoid, alongside smaller extrachromosomal circular DNA fragments called plasmids that often carry antibiotic resistance genes and replicate independently. Genetic information can be exchanged through horizontal gene transfer between bacteria rather than parent to offspring. Transformation is the uptake of free, naked DNA from the environment by a competent cell. Transduction involves the transfer of bacterial DNA from one cell to another via a bacteriophage. Conjugation is the direct transfer of DNA, often a plasmid, between two bacteria connected by a sex pilus or mating bridge.
Bacterial growth in batch culture follows a predictable four-phase curve. During the lag phase, cells adapt to their environment, synthesizing enzymes and metabolites with little to no cell division. The log (exponential) phase follows, in which cells divide at a constant maximal rate and the population doubles at regular intervals called generation or doubling time. As nutrients deplete and waste accumulates, growth rate equals death rate in the stationary phase, where population size plateaus. Finally, the death (decline) phase occurs as cells die faster than they divide. Most bacteria require a carbon source, a nitrogen source, an energy source, water, minerals, and appropriate pH, temperature, and osmotic conditions. Autotrophs synthesize organic compounds from CO2, while heterotrophs obtain organic carbon from other organisms. Mesophiles, which grow between roughly 20-45 degrees C, include most human pathogens because this range encompasses body temperature.
Bacteria vary widely in their oxygen requirements. Obligate aerobes require oxygen for growth, while obligate anaerobes are killed or inhibited by its presence. Facultative anaerobes can grow with or without oxygen, preferring aerobic respiration when available. Aerotolerant anaerobes do not use oxygen but tolerate its presence, and capnophiles, such as Neisseria gonorrhoeae, grow best with elevated carbon dioxide. Some organisms, termed extremophiles, are adapted to harsh environments, including thermophiles (heat), psychrophiles (cold), halophiles (high salt), and acidophiles (low pH). In anaerobic conditions without an electron transport chain, many bacteria perform fermentation, breaking down substrates such as glucose while using an organic molecule as the final electron acceptor. Biofilm formation, in which structured bacterial communities embedded in a self-produced matrix adhere to surfaces like catheters and implants, is clinically important because biofilms dramatically increase antibiotic resistance and underlie many chronic infections.