Monday, 24 September 2012

Carbenicillin, a satellite antibiotic with first team potential

Exactly two weeks ago I posted information on Ampicillin as part of a new mission of mine, i.e., talking a little more about the antibiotics I work with in the lab. It's not only a good way of letting my audience know about a very interesting subject that usually just stands there in front of our eyes and remotely in our heads (because we drop a few microliters in the medium and put it back in the fridge), but also a tremendous opportunity to learn a little bit more about the things I deal with on a daily basis. 

You know, I have a terrible stubbornness in what concerns information. If I work with something, I need to know the ins and outs of what I am doing. I cannot operate as a mere robot, just performing a certain series of activities and not knowing exactly the purpose. That kind of lenient attitude drives me crazy. My approach, on the other hand, has bad and good points... I know a lot of what I am doing but I also invest precious time in getting to know it. Thus, this blog is the right place and the right vehicle to simultaneously learn and help learning.

So, last time I gave it a go on Ampicillin. Today I am going to offer you a good substitute for Ampicillin in a Microbiology Laboratory - Carbenicillin.

Carbenicillin is a bacteriolytic agent that works basically the same way as Ampicillin. This antibiotic belongs to a subgroup of Penicillins, just like Ampicillin, however Carbenicillin is a Carboxypenicillin, along with Ticarcillin (an injectable antibiotic famous in the treatment of Pseudomonas aeruginosa and Stenotrophomonas maltofilia). Some researchers actually consider Carbenicillin to be a lot more effective than Ampicillin and recommend it as a nice substitute for Ampicillin. There is a tremendously good PDF file on the different points where these two antibiotics overcome each other (find it here). Structurally, the main difference between these two antibiotics is that Ampicillin is an aminopenicillin (6-aminopenicillanic acid consisting of a ß-lactam ring linked to a thiazolidine ring and a side chain) whehereas Carbenicillin is a penicillin with a carboxyl and benzyl group.

Ampicillin versus Carbenicillin 

Carbeniccilin is more stable than Ampicillin when used in growth media, as for example LB Broth. Shows better resistance to heat shock and better resistance to degradation caused by acidic pH. Satellite colonies, are colonies that usually grow around an antibiotic resistant colony because the later release enzymes that degrade the antibiotic, thus benefiting the colonies sitting in the vicinity of the resistant ones (usually you can differentiate resistant and non-resistant just by conferring the size). Satellite colonies are a common problem in media containing Ampicillin, but with Carbenicillin these satellite colonies that are formed in Ampicillin plates due to beta-lactomases destroying the antibiotic in the outer cell moiety, are unlikely to appear. Why? Well, Carbenicillin is less unstable in the presence of beta-lactomases, thus reducing the likelihood of satellite colonies. Dead simple! Another important fact is that Carbenicillin is usually cheaper than Ampicillin. I couldn't confirm this price theory as I've only visited one website, and the one website I have visited is selling 5g of Carbenicillin for 110 American Dollars or 70 Sterling Pounds (80 Euros) [1]. The following image was taken from the PDF mentioned (and linked) priorly.

Carbenicillin's stock and working concentration should be 100mg/ml and 100mg/L, respectively. The aqueous solution, as any aqueous solution is short-lived and storage at -20C is a must. Some people prepare it with ethanol to avoid having to thaw it whenever they need to use it. What I personally do is to prepare it with distilled water and aliquot it into 2ml stock tubes.

Now very briefly - an immediate danger associated with injection of Carbenicillin is Hypopotassaemia (low blood potassium as Carbenicillin promotes loss of potassium at the kidney level). Anyway, you are not supposed to be medicating yourself so just take this as plain information.

Don't forget!!! There is a very good OLD paper from 1977 counterpoising Carbenicillin to Carfecillin (Carbenicillin's alpha-phenyl ester). For more details see below Baxter et al., 1977 [2].

[1] Carbenicillin direct, [http://www.carbenicillin.com/order.htm], last visited on the 24th of September 2012, last update unknown.

[2] Basker, M. J., Comber, K. R., Sutherland, R., Valler, G. H. (1977). "Carfecillin: antibacterial activity in vitro and in vivo". Chemotherapy, 23(6), pp. 424-435.

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