The antibacterial properties of plant volatile oils have been recognised since antiquity and have been rediscovered in more recent times. Eucalyptus leaf oils have received attention in a number of studies. Deans and Ritchie () examined the antibacterial effects of fifty volatile oils purchased from a commercial supplier, including eucalyptus, on twenty-five different bacterial genera. The culture collection consisted of food spoilage, food poisoning, human, animal and plant disease types, along with indicators of faecal pollution and secondary opportunist pathogens. Eucalyptus oil was most effective against Elavobacterium suaveolens and the dairy organism Leuconostoc cremoris. However, it was not amongst the ten most inhibitory oils (thyme, cinnamon, bay, clove, bitter almond, lovage, pimento, marjoram, angelica and nutmeg).
Leaf oils from eight Brazilian-grown eucalypts were tested against Mycobacterium avium by Leite et al. (): E. botryoides, E. camaldulensis, Eucalyptus citriodora, E. deglupta, Eucalyptus globulus, E. grandis, E. maculata and E. tereticornis. M. avium was sensitive to all the oils at 10mg/ml but only four of them at 5 mg/ml: Eucalyptus citriodora, E. maculata, E. camaldulensis and E. tereticornis. Eucalyptus citriodora and E. maculata oils were particularly rich in citronellal and citronellol. Eucalyptus citriodora was also one of the better oils tested by Hajji et al. () against bacteria such as Bacillus megaterium and Staphylococcus aureus although, like most of the other oils, it was least effective against Escherichia coli.
Another screening programme, this time involving seventeen eucalypts growing in Uruguay, had earlier been conducted by Dellacassa et al. (). Tests were carried out against two Gram-positive bacteria (Bacillus subtilis and S. aureus) and two Gram-negative ones (E. coli and Pseudomonas aeruginosa) using volatile oils from the following Eucalyptus species:
E. affinis, E. amplifolia, E. botryoides, E. camaldulensis Eucalyptus citriodora, E. cladocalyx, E. diversicolor Eucalyptus globulus, E. hhmanni?, E. longifolia, E. maculata, E. melliodora, E. paniculata, E. pellita, E. punctata, E. sideroxylon, E. tereticornis
B. subtilis and S. aureus were most sensitive to the oils and P. aeruginosa least sensitive. For the latter, oils from only six of the eucalypts showed any inhibition of its growth. Of the seventeen eucalypts in all, only three (E. affinis, E. cladocalyx and E. diversicolor) inhibited the growth of all four organisms. Overall, oil from Eucalyptus globulus (which had the highest cineole content, 64.5 per cent) was the least effective, only showing some activity towards S. aureus. Eucalyptus citriodora oil only inhibited S. aureus and E. coli. The authors reported no correlation between either 1,8-cineole content or the content of any other constituent and antimicrobial activity, and suggested that the observed activity was due to combinations of more than one oil constituent that are specific to each test bacterium.
In yet another screening study, Kumar et al. () evaluated freshly distilled leaf oils from twenty-four species of Eucalyptus against eight Gram-positive and seven Gram-negative bacteria, some of which were pathogens. The eucalypts were all growing locally in India but unfortunately no information was provided on the chemical composition of the oils:
E. camaldulensis (X2), E. crebra, E. dalrympleana, E. deglupta Eucalyptus globulus, E. goniocalyx, E. grandis, ‘E. kirtoniana’, E. laevopinea, E. leucoxylon, E. melanophloia?, E. microcorys, E. parvifolia? E. regnans, E. robertsonit, E. robusta, E. rubida, E. rudis, E. tereticornis, E. viminalis, E. alba X E. camaldulensis, E. tereticornis X E. camaldulensis, ‘Eucalyptus hybrid’
The Gram-positive bacteria tested included Bacillus anthracis, the causative organism of anthrax, B. subtilis, Micrococcus glutamicus, Sarcina lutea, Staphylococcus aureus and Streptococcus pyogenes. The Gram-negative bacteria included Enterobacter sp., Listeria monocytogenes, Proteus vulgaris and Pseudomonas sp. The authors confirmed that in general Gram-negative bacteria are less susceptible than Gram-positive ones. As might be expected, there were differences in inhibitory powers between the oils: E. tereticornis, one of the E. camaldulensis samples and E. grandis were effective against thirteen of the fifteen organisms but E. melanophloia and ‘Eucalyptus hybrid’ showed no inhibition at all towards any of the Gram-negative bacteria. However, the lack of any compositional data on the oils is a serious weakness in the work – there were marked differences in the effectiveness of the two E. camaldulensis oils, for example, for which no explanation is offered. Of the Gram-positive bacteria, Micrococcus glutamicus was the most sensitive while Streptococcus pyogenes and Sarcina lutea were the most resistant, fewer than half the oils inhibiting them.
In a study of Malagasy medicinal plants De Medici et al. () analysed oil from the leaves of Eucalyptus citriodora and Eucalyptus globulus, along with an undefined Eucalyptus species. The oils were also tested for activity against Escherichia colt. Using undiluted oils, Eucalyptus citriodora oil (71 per cent citronellal) proved to be inactive while Eucalyptus globulus (40 per cent cineole) and Eucalyptus sp. (18 per cent cine-ole, 43 per cent α-pinene) were only weakly active. Comparative results of Eucalyptus citriodora and Eucalyptus globulus oils with those of cineole-rich Cinnamomum camphora and Melaleuca viridiflora, and eugenol-rich Ocimum gratissimum.
Eucalyptus citriodora oil had earlier been tested against ten bacteria by Siva Sankara Rao and Nigam (). It showed some activity towards eight of them (Bacillus fumilis, Micrococcus sp., Pseudomonas solanacearum, Sarcina lutea, Staphylococcus albus, Staphylococcus sp., Shigella sp. and Xanthomonas campestris) but not to Erwinia carotovora and Pseudomonas mangifera indica. It was not as effective as the oil from Cinnamomum zeylanicum.
The difficulties in trying to account for the activity of whole oils simply by considering the activity of individual constituents in isolation has been well demonstrated by Low et al. (). They showed that artificial mixtures of citronellal/citronellol/cineole or citronellal/citronellol in the same concentrations as found in their sample of Eucalyptus citriodora oil (i.e. 90 : 7.5 : 2.5 or 90 : 7.5, respectively) were as effective as the oil itself against Staphylococcus aureus and Salmonella typhi. This was in contrast to citronellal, citronellol or cineole individually.
The work of Chaudhari and Suri () on E. tereticornis and E. camaldulensis hybrids and their parents, referred to earlier in connection with antifungal activity, included tests against eight bacteria: Bacillus mycoides, B. pumilus, Escherichia coli, Proteus vulgaris, Salmonella paratyphi, Sarcina lutea, Shigella nigesta and Staphylococcus aureus. The four oils were active against all the organisms at a dilution of 1: 500.
Production of a soap in which the fragrance also serves as a bacteriostatic agent would have several advantages and Morris et al. (), in the laboratories of International Flavors and Fragrances, investigated over 520 fragrance raw materials with this objective in mind. The test samples included both whole oils and pure aroma chemicals and were initially screened against Escherichia coli, Staphylococcus aureus and Candida albicans. Some 44 per cent of the samples were inhibitory towards a single organism but only 15 per cent were active against all three. A lipophilic diphtheroid, Corynebacterium sp., was added to the testing protocol for just over 200 of the original samples, including a commercial cineole-rich eucalyptus oil (70–75 per cent). For eucalyptus oil, as for many of the other materials, the tests gave disappointing results: using the paper disc diffusion assay there were no zones of inhibition against any of the four test organisms and MIC values were S’1000ppm. Even the most promising samples had MICs inferior to that of a commonly used commercial soap bacteriostat and when tested in soaps no reduction of bacterial counts was obtained. The authors concluded that it did not appear to be possible to produce a practical antimicrobial soap fragrance. They also pointed out the need for caution when using experimental zones of inhibition as a measure of antimicrobial activity. Such a method is dependent on the solubility and rate of diffusion of the test sample in the aqueous medium and a sample may be much more bacteriostatic than its zone of inhibition might indicate. One form of testing which is closer, in practical terms, to the way in which human infections are often spread, namely as bacterial aerosols through coughing and sneezing, has been investigated by Chao et al. (). Thieves is a proprietary blend of five volatile oils: Eucalyptus globulus, Cinnamomum zeylanicum, Citrus Union, Rosmarinus officinalis and Syzygium aromaticum. In well-designed experiments subjected to statistical analysis, the blend’s antibacterial activity was tested against three Gram-positive organisms Micrococcus luteus, Pseudomonas aeruginosa and Staphylococcus aureus bioaerosols. Thieves was allowed to diffuse into an enclosed fume hood following spraying of the aerosol-borne bacterial load. P. aeruginosa was the most sensitive to the treatment, with a 96 per cent reduction in bacterial count following a 10-min exposure. Inhibition levels for M. luteus and S. aureus were 82 per cent and 44 per cent, respectively. With further testing the authors suggest that commercial applications could include diffusion of oil aerosols into air-conditioning systems and spraying in enclosed rooms, both at work and at home, to prevent the transmission of illnesses through air-borne bacterial pathogens.