The dreaded API 20E Biocode strip. Don’t get me wrong; these things are great for identifying bacteria and not having to memorize the entire biochemical work up. (Believe me, that is torture I am glad is over.) The only downside is how often they don’t work if they are not incubated or inoculated correctly.
The premise behind them is they are basically a portable, small-scale biochemical reaction vessel. The “old-school” way is to inoculate test tube racks filled with various tests and then read them 24 hours later. API makes it easier and allows you to use a smaller quantity of bacteria to perform the testing. Once you prepare a saline solution of 2-3 well isolated colonies, you fill the cupules up to the edge or completely fill the ones boxed in. A few will also be underlined and have to be covered with sterile mineral oil in order for the reaction to take place.
I can remember many times, especially during our “other gram -ve” microbiology lab, where the biocodes would not identify the correct organism AT ALL or it wouldn’t be valid for 48 hours. A little challenging when you only have 2 days of Micro labs and cannot incubate the strip over the weekend.
Another challenge is remembering that VP turns a pale pink after 10 minutes of reaction time after alpha napthol and KOH are added. THIS IS NOT A POSITIVE. The test must turn bright magenta pink within a few minutes to be positive. This is easily confused and often trips people up on practical exams (And by people I mean me. Thankfully I realized before I handed in my paper.)
Also, don’t get me started on the API 20NE strips. Those still give me night terrors trying to figure out if the cups are “cloudy” or not.
Read more about API here.