The term “germs” is a non-scientific umbrella term for microorganisms like bacteria, fungi, and viruses. Biologically, these are referred to as microbes, and their collective community in and on the body is known as the human microbiome. The human body hosts trillions of these microbes, many of which are harmless and perform functions beneficial to human health. Answering the question requires differentiating between the sheer total number in a location versus the density per unit area.
The Body’s True Microbial Headquarters
The large intestine, or colon, is the location for the largest total number of microbes, serving as the body’s microbial headquarters. This internal environment houses an estimated 75 to 200 trillion individual microorganisms. This immense population, composed primarily of bacteria, can outweigh the number of human cells in the body.
The colon provides an environment perfectly suited for microbial proliferation. It is a warm, stable, and nutrient-rich habitat that is largely anaerobic, meaning it lacks free oxygen. This anaerobic environment favors the growth of dominant gut bacteria like Bacteroides and Firmicutes. These microorganisms perform functions the human body cannot, such as fermenting undigested dietary fibers.
This fermentation generates short-chain fatty acids (SCFAs) like butyrate, which is a major energy source for the cells lining the colon. The microbes also compete for space and resources, which helps prevent the colonization of harmful, disease-causing bacteria. The bacterial concentration in the colon can exceed $10^{11}$ to $10^{12}$ organisms per gram of intestinal content, making it one of the densest microbial communities on Earth.
External Hotspots of Microbial Life
While the gut hosts the highest total count, external surfaces feature several hotspots where microbial density is extremely high in localized areas. The mouth is one of the most diverse and densely populated external environments, rivaling the gut in species richness. Constant moisture and a steady supply of nutrients from food and saliva allow bacteria like Streptococcus mutans and Veillonella parvula to thrive on the teeth, gums, and tongue.
The skin has a much lower total microbial count than the gut but exhibits massive variations in density depending on the anatomical site. Areas that are occluded, moist, and warm provide the best conditions for microbial growth. This includes the armpits, groin, and between the toes, where high humidity and temperature lead to elevated populations of Staphylococcus and Corynebacterium species.
Conversely, dry, exposed skin areas like the forearms have a lower density, favoring species that tolerate desiccation. The forehead and upper back, which are rich in oil-producing sebaceous glands, also show high microbial density. The feet, especially when covered by socks and shoes, create a warm, dark, and moist environment. This encourages a high load of bacteria, contributing to the characteristic odor associated with foot sweat.
Environmental Conditions That Dictate Density
The distribution of microbes across the body is determined by a few universal environmental principles. The availability of moisture is the most important factor, as microbes need water for metabolic processes and growth. Consequently, wet areas consistently harbor more organisms than dry ones. Temperature is another major determinant, with the internal body temperature of $98.6^{\circ}F$ being ideal for most commensal microorganisms.
The availability of oxygen is also a defining factor, creating two distinct types of microbial communities. External surfaces are generally aerobic, supporting microbes that require oxygen. Deep internal sites like the colon are anaerobic, favoring organisms that thrive without oxygen. Finally, a constant nutrient supply—whether digested food, dead skin cells, or oils—is necessary to sustain any densely populated microbial community.