An Overview of Infectious Agents

The Five Kingdoms of Living Things

Biologists generally classify living organisms into one of the five kingdoms illustrated here. Bacteria are the most primitive and likely represent the earliest living organisms, from which the protista and other kingdoms are likely to have evolved. The overlap of the kingdoms in this figure is intentional, because, given the evolution of increasingly complex and diverse species, there are no clear-cut dividing lines, and classification has sometimes been ambiguous. Fungi, for example, were once classified with plants, but some of their structural characteristics are quite distinct from those of plants, and they are now classified in their own kingdom.

Species from all five of the kingdoms have the potential to influence human health, either positively or negatively.

While natural selection implies competition among and within species, we are increasingly aware that there is a strong interdependence among species. For example, most bacteria are non-pathogenic and live on the inner and outer surfaces of our bodies in staggeringly large numbers. These "normal flora" actually outnumber the cells in our body, and they provide many benefits. A key benefit is that by living on our skin and on the epithelial lining of our respiratory, digestive, and uro-genital tract, these usually harmless bacteria prevent pathogenic species from gaining a foothold.

Prokaryotes versus Eukaryotes

The bacteria are the oldest and simplest living organisms, and all of the bacteria are "prokaryotes," meaning that they do not have a true membrane-bound nucleus as eukaryotes do. [Prokaryote is derived from Greek,meaning "before nucleus"; eukaryote means "true nucleus."]

The figures and tables below provide a comparison of prokaryotic versus eukaryotic cells.











Generally smaller (0.2-2.0 μm)

Generally 10-100 μm

No nuclear membrane. There is generally a single circular chromosome composed of DNA

Have a true nucleus, consisting of nuclear membrane & nucleoli. Eukaryotes have multiple linear chromosomes.

No membrane-enclosed organelles.

Membrane-enclosed organelles include lysosomes, Golgi complex, endoplasmic reticulum, mitochondria & chloroplasts

Flagella are simple, consisting of just two protein building blocks

Flagella are complex and are composed of multiple microtubules

Present as a capsule or slime layer

Present in some cells that lack a cell wall

Frequently have a cell wall and a cell cell membrane. The cell membrane lacks carbohydrates and generally lacks sterols

Usually do not have a cell wall. The cell membrane does have sterols and carbohydrates that serve as receptors.

Lack a cytosketeton

Have a cytoskeleton and can perform cytoplasmic streaming

Binary fission



 Toggle open/close quiz question

Which of the following is true about eukaryotic cells?





 Toggle open/close quiz question

Which of the following is NOT true about prokaryotic cells?