Bacteria are the most ancient and abundant life forms on Earth. Estimates suggest the planet may host up to a trillion microbial species, with the vast majority remaining undiscovered. These single-celled organisms are ubiquitous, inhabiting every environment from the deepest oceans and solid rock to the surfaces and interiors of all multi-cellular life. Understanding these organisms is fundamental to grasping the mechanics of the biosphere, as they drive global processes and directly influence the health of every other living thing.
Defining Characteristics of Bacteria
Bacteria are defined structurally as prokaryotes, meaning they are single-celled organisms lacking a membrane-bound nucleus and other internal compartments common to animal and plant cells. Their genetic material, typically a single, circular chromosome of double-stranded DNA, is concentrated in a region called the nucleoid, floating freely within the cytoplasm. This simple cellular architecture does not include organelles like mitochondria or the endoplasmic reticulum.
The cell is protected by a rigid cell wall, which in most bacteria contains a polymer called peptidoglycan that maintains the cell’s shape and prevents it from bursting due to osmotic pressure. Many bacteria also possess small, extra-chromosomal rings of DNA known as plasmids, which can be easily transferred between cells and often carry genes that provide an advantage, such as antibiotic resistance. Most bacteria range in size from 0.5 to 5 micrometers and reproduce rapidly through binary fission, where a single cell duplicates its DNA and divides into two identical daughter cells.
Essential Roles in Ecosystems
Bacteria perform functions that are indispensable for maintaining life on Earth, operating as the planet’s primary recyclers. As decomposers, they break down complex organic matter from dead plants and animals into simpler inorganic compounds, releasing nutrients back into the soil and water. This mineralization process allows elements like carbon and phosphorus to be continuously recycled and made available for new life to grow.
Their specialized metabolic capabilities are particularly apparent in the global nitrogen cycle, which would cease to function without them. Atmospheric nitrogen, or $\text{N}_{2}$, is inert and unusable by plants, but certain bacteria, such as Azotobacter, perform nitrogen fixation, converting this gas into ammonia. Other bacterial groups then continue the process, with nitrifying bacteria converting ammonia into nitrites and nitrates, forms that plants can readily absorb.
Bacteria also form a beneficial relationship with the human body, particularly within the gut microbiome. This vast community of microbes ferments non-digestible dietary fibers into short-chain fatty acids (SCFAs), such as butyrate, which serves as the main energy source for the cells lining the colon. Gut bacteria synthesize several vitamins that the body cannot produce on its own, including Vitamin K and various B vitamins like folate and riboflavin. This microbial community also plays a role in training the immune system, helping it distinguish between harmless foreign substances and actual threats.
Bacteria and Human Disease
While many bacteria are beneficial or benign, a small fraction possess specific mechanisms, known as virulence factors, that enable them to cause human disease. Pathogenic bacteria operate primarily through two mechanisms: direct tissue invasion and the production of toxic molecules. Tissue invasion involves the production of enzymes, such as collagenase, which allow the bacteria to break down the host’s connective tissues, enabling the infection to spread deeper into the body.
The second major mechanism involves toxins, which are biological poisons produced by the bacteria. These toxins are broadly categorized as exotoxins and endotoxins. Exotoxins are proteins secreted by the living bacterial cell, and they can be harmful even in minute quantities, such as the botulinum toxin produced by Clostridium botulinum. Endotoxins are components of the outer membrane of Gram-negative bacteria, known as lipopolysaccharide (LPS), which are released when the bacterial cells die or divide.
The rapid proliferation of bacteria also contributes to disease severity, as a small number of invading cells can quickly multiply and overwhelm the body’s defenses. A common example of a localized bacterial infection is strep throat, caused by Streptococcus pyogenes. In severe cases, the unchecked spread of bacteria into the bloodstream can lead to septicemia, a systemic infection that can result in shock and organ failure.
The Ongoing Battle Antibiotics and Resistance
The development of antibiotics revolutionized medicine by providing targeted chemical agents to combat bacterial infections. Antibiotic drugs function by interfering with structures or processes unique to bacterial cells, typically by inhibiting cell wall synthesis, disrupting protein production, or blocking DNA replication. For instance, a common class of antibiotics prevents bacteria from building the peptidoglycan layer, causing the cell to rupture.
However, the intensive use of these drugs has driven a rapid evolutionary response in bacterial populations, leading to the public health challenge of antibiotic resistance. Bacteria achieve resistance through various mechanisms, often encoded on transmissible plasmids, allowing them to evade the effects of the medication. One common strategy involves bacteria producing specialized enzymes, such as beta-lactamases, which chemically inactivate the antibiotic molecule before it can reach its target.
Other resistance mechanisms include altering the target site of the drug, so the antibiotic can no longer bind effectively, or employing efflux pumps, which are membrane proteins that actively pump the drug out of the bacterial cell before it accumulates to a toxic level. This ongoing evolution means that previously effective drugs are becoming increasingly obsolete, forcing researchers to constantly seek new ways to treat bacterial diseases.