Which of the following strain types differs in their antigenic determinants?

The correct answer is serovar. Serovars are distinct variations within a species of bacteria or virus that are classified based on the antigenic properties of their surface structures. These antigenic determinants, such as proteins or carbohydrates present on the cell surface, can elicit different immune responses in a host. This specificity allows researchers and healthcare providers to identify and differentiate between serovars based on serological techniques.

In contrast, a strain refers to a genetic variant or subtype of a microorganism that may not necessarily have distinct antigenic properties. All strains within a given serovar typically share the same antigenic structures, but they may exhibit other genetic differences. A species is a broader classification that encompasses a group of organisms that can interbreed; it may include multiple serovars and strains. An isolate is a sample of an organism that has been obtained from a specific environment or patient and may not imply any antigenic characteristics itself, as it is more about the context of the collection rather than the specific antigenic properties.

Thus, serovar is the term that specifically denotes variations in antigenic determinants among microorganisms, making it the correct answer.