In a laboratory culture, what are plaques indicative of when cells are infected by a virus?

When observing plaques in a laboratory culture where cells are infected by a virus, the primary indication is that the virus has spread and left behind dead cells. Plaques are formed due to the lytic cycle of a virus, where viral replication leads to the destruction of the host cells. As the virus infects neighboring cells, those cells also eventually succumb to viral replication, resulting in localized areas of cell death.

This phenomenon is particularly important in virology as it provides a method to quantify viral infectivity and study the dynamics of viral spread in a controlled environment. The appearance of these plaques helps researchers understand how effectively the virus can replicate and spread within a host environment, underscoring the destructive relationship between a virus and its host cells.

In this context, the other choices do not accurately describe what plaques indicate. For instance, while viral replication is an essential aspect of the process, the specific observation of plaques is more closely related to the outcome—dead cells—rather than simply replication status. Healthy cells or the absence of bacteria would not result in plaque formation, as plaques specifically represent areas where cell death has occurred due to viral activity.