Group A Streptococci

00:00 and then you can blame the food handlers.

00:04 Alright those are the staphylococci, let’s move to another coccus, streptococci, also gram-positive cocci, growing in pairs or chains as you can see in this micrograph of the organism.

00:17 We put them into two groups, there are a number of different ways to categorize streptococci, so if you decide by listening to this lecture, you want to go read about streptococci, you may find other classifications and you will say, Racaniello didn't know what he's talking about, but there are just many ways to do it and so I'm going to tell you one way because we can't tell you all ways and that is as follows. There are group A streptococci, which cause a wide range of clinical diseases and then there are group B, which are leading causes of neonatal sepsis, that is disseminated infections and meningitis. I like this because it's a very simple classification and you can remember it. Just like the staphylococci, many people carry streptococci in them as well, this is ubiquitous in the human population, it is worldwide, it's in every country and it's found on the skin and the nasopharynx, your nose and pharyngeal tissues of about 20% of school-aged children. So if you go to any random classroom in the world, about 20% of those kids have these bacteria, the streptococci on and in them. And when you do carry streptococci, usually you're okay, we say you're asymptomatic, you're a carrier and you could potentially infect other people, but you are okay and this is a common occurrence. And when infection and disease occurs, it's usually in the form of say, pharyngitis, and this happens in school children. So you may have a class full of kids, 20% of them have streptococci, you put them together, they infect other kids who don't have the bacteria, they may get pharyngitis, we also call it strep throat. This is a rather painful throat infection and if you get a throat infection and it's really hurting, it's good possibility that it's strep throat caused by streptococci.

02:14 The bacteria may also infect the skin and soft tissues, causing lesions that are called pyodermas. You can see one here on the young lady's arm. Pyodermal infections usually require a break in the skin to introduce the bacteria, similar to the story with staphylococci, alright, so in the pharynx you don't require a break, but on the skin here, a break in the skin, if you're a carrier, you get an infection, if you're not a carrier and you are unlucky enough to have touched someone who has streptococci on your skin you'll get a pyoderma. It spreads from person to person, again by respiratory droplets produced by coughing, sneezing or talking or by direct contact, very much like the staphylococci.

02:59 Most of these streptococci remain localized. Either in your throat, if you have pharyngitis or on your skin, a pyoderma, but over the last 10 to 20 years, we have seen an increase in strains of streptococci that apparently can invade deeper tissues and this is serious and a worrisome occurrence, because these are life-threatening infections. So the bacteria not just staying in the throat or on the skin, but they're invading, they're moving systemically and as you can see, all the different organs in this young lady can be infected and these are serious infections.

03:37 Group A streptococci produce proteins that promote spread of the bacteria, these are exotoxins that we've talked about before and include proteases that digest proteins in the host which would normally restrict the spread of the bacteria, hyaluronidases. Hyaluronic acid is a component of the extracellular matrix and this is digested by hyaluronidase, it loosens up tissues and allows the bacteria to spread and DNAses, these often extracellular DNA in tissues that makes it very thick and it's difficult for bacteria to move through this, they make a DNAse which chops up the DNA and allows free spread of the bacteria.

04:20 These bacteria also produce an enzyme called streptokinase, it's shown here in three-dimensional structure, the protein. This enzyme converts plasminogen to plasmin. Plasmin in turn degrades fibrin. Fibrin is a component of blood clots and it restricts bacterial movements, so this is another strategy the bacteria have to move around the body. Bacteria also produce streptolysins, two different kinds, S and O, these lyse cells of various sorts, including immune cells that are trying to get rid of the bacteria, so you can see the value of that for the bacteria.

04:59 And the bacteria also are very good at avoiding phagocytosis, remember macrophage-like cells are trying to take up these bacteria to destroy them, the bacteria have on their surface an M protein, it forms a dense layer on the surface of the bacteria, it binds complement and inhibits uptake by the macrophages. The bacteria also have a capsule on the outer surface; this is another anti-phagocytic structure, so it has at least two ways to avoid phagocytosis and destruction.

05:30 Some of the more serious outcomes of streptococcal infections, in particular group A streptococcal infections, are non-suppurative sequelae, this means fever without pus. Now you remember an abscess is full of pus, dead neutrophils and other things, but there can also be fever and infection without pus and that's what non- suppurative sequelae means. One of the more serious is acute rheumatic fever. This is a syndrome that can occur from one to four weeks after the initial infection, so a child gets strep throat and if you have a child that gets strep throat, you should worry immediately that this sequelae is going to occur, rheumatic fever, because it can be life-threatening way beyond the sore throat, so get the infection treated. This infection involves carditis, infection of the heart, polyarthritis, infection of many joints, chorea, subcutaneous nodules and erythema marginatum. The inflammation of the heart is the most serious issue here; this can cause scarring of the heart valves.

06:38 So these bacteria infect the heart, the valves get scarred and for the rest of the child's life, they have heart issues because of this and this can also kill them, it can be a fatal infection. Why does this happen? It happens to be an autoimmune disease. We make antibodies against the bacteria as they're growing in us, but some of those antibodies also recognize our heart tissue and begin to attack heart tissue and destroy it. This doesn't happen in everyone and it's probably a consequence of your particular immune makeup, but that's why it's an autoimmune disease, rheumatic fever after infection with group A streptococci.