Final answer:
Harmful bacterial cells to the body are known as pathogenic. They can produce diseases and toxic substances, contrasting with beneficial, commensal, or normal flora bacteria, which are usually harmless or helpful.
Step-by-step explanation:
Bacterial cells that may be harmful to the body are known as pathogenic. These microbes are capable of causing diseases and can produce toxic substances that are detrimental to human health. An example of a pathogenic bacterium is certain strains of Escherichia coli, which can cause serious diarrheal disease. Although most E. coli are commensal and beneficial, the pathogenic types have acquired virulence factors like type 1 fimbriae or toxins through horizontal gene transfer, enabling them to colonize and damage the host. Moreover, the relationship between humans and pathogenic bacteria is often parasitic since these bacteria invade the body and promote illness, exemplified by diseases such as tetanus, tuberculosis, and leprosy.
It's important to distinguish pathogenic bacteria from beneficial, commensal, and normal flora, which are types of bacteria that typically pose no threat and can even provide health benefits. In fact, our bodies contain a diverse community of these benign microbes that aid in digestion, vitamin synthesis, and protecting us against harmful bacteria. Biological barriers, including a population of harmless bacteria, help defend against pathogenic species by occupying space, consuming nutrients, changing environmental conditions, and sometimes secreting substances that inhibit the growth of harmful bacteria.