IDENTIFICATION OF GRAM POSITIVE ORGANISM: AN EASY APPROACH

Added: 2026-05-06

[00:00:00]Hello everyone, welcome to Lab Science Tutorials. In today's video, I'll be giving you a master chart for the identification of gram-positive organism.

[00:00:11]If you're an aspirant of AIIMS, heart, DHA, or any other competitive exam, you know that bacteriology is a very important session. You will get a lot of questions from bacteriology on the identification of organism. So, this chart will help you revise the entire group of gram-positive organism in just a few minutes. So, here we go.

[00:00:55]Gram-positive bacteria appear purple or blue in a gram stain. We can classify gram-positive bacteria into three categories: bacilli, cocci, and branching filaments. First, let's look at gram-positive bacilli. Remember, bacilli are slender or rod-shaped, cocci are spherical or round-shaped, and branching filaments form branched structures. Gram-positive bacilli can be further classified as aerobic or anaerobic. The main aerobic gram-positive bacilli are Listeria, Bacillus, and Corynebacterium. The anaerobic ones are Clostridium and Cutibacterium. So, if you get an exam question that says a patient was admitted to the hospital, a sample was collected, gram staining was done, and it showed gram-positive aerobic bacilli, then think about Listeria, Bacillus, and Corynebacterium. If the question says gram-positive anaerobic bacilli, then think Clostridium or Cutibacterium.

[00:01:56]This is how you identify gram-positive bacilli. Now, let's talk about gram-positive branching filaments.

[00:02:04]Branching filamentous bacteria can also be aerobic or anaerobic. The main example of a gram-positive aerobic branching filament is Nocardia. Nocardia is also weakly acid-fast in nature.

[00:02:18]That's an important point to remember.

[00:02:21]On the other hand, anaerobic branching filaments are not acid-fast, uh and the example is Actinomyces.

[00:02:44]Now, let's discuss gram-positive cocci, which is a very, very important group of organisms. How do you differentiate gram-positive cocci? First, we do a catalase test. If the catalase test is positive, the bacteria are Staphylococcus. If catalase is negative, then it could be Streptococcus. Now, we need to identify which Staphylococcus species it is. For that, the next test is coagulase test. If coagulase is positive, then it is Staphylococcus aureus. If coagulase is negative, we have to think about other group, which is coagulase-negative staphylococci or CONS. The main CONS are Staphylococcus saprophyticus and Staphylococcus epidermidis. To differentiate them, uh we have uh novobiocin sensitivity test. If the organism is sensitive to novobiocin, it is Staphylococcus epidermidis. If it is resistant to novobiocin, it is Staphylococcus saprophyticus. So, when you get an exam question, remember these key points. If the question says gram staining shows gram-positive cocci that are catalase-positive, positive, then you know it is Staphylococcus, not Streptococcus. If it is says coagulase-positive, then you know it is Staphylococcus aureus.

[00:04:08]Coagulase-negative, then the next differentiation point is novobiocin.

[00:04:14]Novobiocin-sensitive is Staphylococcus epidermidis. Resistant is Staphylococcus saprophyticus.

[00:04:34]Now, let's discuss Streptococcus.

[00:04:37]If it is a gram-positive bacterium and it is catalase-negative, then it belongs to the Streptococcus group. How do we differentiate the different streptococci? The major identification is by hemolysis on blood agar.

[00:04:52]When streptococci is grown on blood agar, it shows different types of hemolysis. Partial hemolysis, we call it alpha hemolysis. Complete hemolysis, we call it beta hemolysis. No hemolysis, then we call it gamma hemolysis. If there is alpha hemolysis or partial hemolysis, it is usually Streptococcus pneumoniae or viridans streptococci. How do you differentiate them? We use optochin sensitivity and bile solubility. If it shows alpha hemolysis and is optochin-sensitive and bile-soluble, then the organism is Streptococcus pneumoniae, which is a capsulated organism. If it is optochin-resistant and bile-insoluble, then it is viridans streptococci.

[00:05:43]Now, what if it is beta hemolysis or complete hemolysis? Beta hemolysis main is mainly shown by group A and group B streptococci. Group A Streptococcus is Streptococcus pyogenes, and group B is Streptococcus agalactiae.

[00:06:01]We differentiate them using bacitracin sensitivity. If there is beta hemolysis and the organism is bacitracin-sensitive, then it is group A Streptococcus, which is Streptococcus pyogenes. If it is bacitracin-resistant and also CAMP test positive, then it is group B Streptococcus, which is Streptococcus agalactiae. Streptococcus pyogenes is CAMP test negative. Finally, what if there is no hemolysis or gamma hemolysis? Then it could be Enterococcus or non-Enterococcus streptococci. We differentiate them by growth in 6.5% sodium chloride. If there is growth in 6.5% sodium chloride and no hemolysis, then organism is Enterococcus. If there is no growth in 6.5% sodium chloride and no hemolysis, it could be non-Enterococcus streptococci like Streptococcus bovis.

[00:07:05]That's all for today's video. In my next video, we will be discussing gram-negative organism. If you like this video, please do like, share, and subscribe. Also, let me know in the comments which topic you want me to cover in future videos. Thank you so much.