The following information comes from Soraya Gharibi’s PhD project.

Main picture courtesy of Taronga Zoo Mosman, NSW

Compressed Bloo Feb 1

Whole blood and its components such as serum and plasma, with or without platelets, has antimicrobial activity. To measure medicine concentrations in blood, plasma is usually used. Plasma is produced by adding an anticoagulant such as sodium or lithium heparin, citrate or oxalate to whole blood, and after centrifugation at 14 000 g for 10 minutes, the blood separates with clear plasma at the top of the tube and the red blood cells below. Serum, in contrast to plasma excludes fibrinogens (clotting proteins), and is  the resulting upper clearn component when whole blood is allowed to clot and then centrifuged. Constituents of serum and plasma have antimicrobial activity include complement, lysozyme enzymes, antibodies and small antimicrobial peptides (AMPs). Additionally, when the coagulation system is activated it traps bacteria inside the clot as well as producing AMPs.

As eucalypt extracts and essential oils (EOs) are also recognised to have antimicrobial activity in-vitro, this suggests an obvious hypothesis that the almost exclusive eucalypt diet of koalas may have a synergistic effect on the antimicrobial activity of koala plasma and/or serum. Although their diet is based on the ingestion of one specific plant genus, both wild and captive koalas have a preference for a variety of eucalypt species foliage in their diet. The diet of wild koalas may include a diverse range of Eucalyptus species foliage based on tree abundance and species variety within their habitat At Taronga Zoo, captive koalas are fed a variety of Eucalyptus species foliage simultaneously, and such varieties include Eucalyptus amplifolia, Eucalyptus microcorys, Eucalyptus molucanna, Eucalyptus punctate, Eucalyptus robusta, Eucalyptus scoparia and Eucalyptus tereticornis.

Many Eucalyptus species have been studied to identify their chemical composition as well as biological activities. Eucalypt EOs are estimated to account for 16 % of the total digestible energy intake by koalas . The susceptibility of pathogens to a range of EOs from a variety of Eucalyptus species, is reported to vary significantly. For example, EOs extracted from E. viminalis and E. globulus are better at inhibiting the bacteria Staphylococcus aureus (S. aureus) compared to Escherishia coli (E. coli) while E. ovata  and E. robusta  have greater inhibition of E. coli than S. aureus.

To date, little attention has been paid to the therapeutic properties and the effects of eucalypt diet on the susceptibility or resistance of the koala to infectious disease. To the best of this author’s knowledge, there is no published information about the baseline antibacterial activity of koala blood, therefore, this study evaluated the antibacterial activity of koala plasma and to compare  with that of other species.

Soraya did a series of experiments to compare the antimicrobial action of plasma and blood of koalas, sheep and horses to inhibit two bacteria E. coli and S. aureus.  These bacteria species were used as they cause many infections in animals.

On the above figure the amount of bacterial growth is measured by optical density (OD) (on the y axis) at a wavelength of 620 nm. The higher the OD, the more bacterial growth.

In the koala, sheep and horse study, sterile plasma from this species was incubated with a known concentration of bacteria and bacterial growth fluid (agar broth). In the positive control there was only bacteria and broth (no plasma).

The following are what Soraya found:


Bacterial growth of E. coli ATCC 25922 (A) and S. aureus ATCC 29213 (B) when incubated with different plasma samples at 35 °C for 18 – 24 h. Koala plasma samples were from 15 adult koalas. Cow and horse plasma samples were collected from two animals each. All animals were clinically normal and un-medicated. The growth of the pathogens was estimated by measuring the OD620. Each data point is a replicate. The middle line is the mean and, whiskers are the minimum and maximum values (range).

So what was found with this small sample plasma from animals, that koala plasma has more variability to inhibit both bacterial species than in the horse and sheep.

These results surprised us, we expected that koala plasma would be just as effective of inhibiting these bacterial species as the horse and sheep. However we did not look at the plasma of as many horses and sheep as we did for koalas. Our future experiments will be to explore the reason for this variability in antimicrobial activity in the blood of koalas. Understanding why this is this variability, may help us to understand how to improve treatment for the bacterial disease chlamydiosis in koalas.