plural of strain
"The laboratory catalog listed several bacterial strains that were resistant to common antibiotics."
Third-person singular simple present indicative form of strain
"The heavy backpack strains her shoulders as she walks up the steep hill."
In plain English: To strain something means to pull it hard enough that you think it might break.
"Heavy lifting strains his back every time he picks up the boxes."
plural of Strain
"The laboratory tested various strains of bacteria to find the most effective treatment."