What determines the pathogenicity of a bacterium?

The pathogenicity of a bacterium is fundamentally determined by the presence of virulence factors. These factors are specific characteristics that enable bacteria to invade host tissues, evade the immune system, and establish infection. Virulence factors can include toxins, adherence factors that help bacteria attach to cells, enzymes that facilitate tissue breakdown, and mechanisms that inhibit immune responses.

For instance, some bacteria produce exotoxins that damage host cells or disrupt normal physiological processes. Others might possess flagella or pili that enhance their ability to move and adhere to surfaces in the host, further facilitating infection. The combination and effectiveness of these virulence factors directly influence how likely a bacterium is to cause disease, thereby determining its pathogenicity.

The other options do not accurately reflect the primary determinants of a bacterium's ability to cause disease. Oxygen consumption may indicate metabolic processes but does not reveal anything about the bacterium's ability to cause infection. The color of colonies on agar can provide some information about the type of bacteria but not about their pathogenic potential. Size comparisons may highlight structural differences, but they do not inherently relate to the bacterium's capacity to infect a host.