Introduction

Bacterial resistance to currently used antibiotics is becoming a concern to public health (Monroe and Polk, 2000). The development of bacterial super resistant strains is resulting in currently used antibiotic agents failing to end many bacterial infections. For this reason the search is ongoing for new antimicrobial agents, either by the design and synthesis of new agents, or through the search of natural sources for as yet undiscovered antimicrobial agents (Bhavnani and Ballow, 2000). Herbal medications in particular have seen a revival of interest (Chariandy et al., 1999) due to a perception that there is a lower incidence of adverse reactions to plant preparations compared to synthetic pharmaceuticals. Coupled with the reduced costs of plant preparations, this makes the search for natural therapeutics an attractive option.

Prior to European settlement in Australia, the Aboriginal people used a variety of plant medicines to help maintain their health (Barr et al., 1993; Lassak and McCarthy, 1993). One of the most serious health threats was considered infection. Bacterial growth in the hot, often humid climates in which the Aborigines lived provide ideal growth conditions for bacteria. This resulted in the Aborigines searching for antibacterial agents to curb infection and speed up the healing process. In fact, more than 150 plants from nearly 60 widely varied botanical families were used by Australian Aborigines as antiseptic agents (Lassak and McCarthy, 1993). For example, Aborigines from Groote Eylandt in the Northern Territory of Australia used a warm infusion of the Crinum asiaticum bulb to disinfect wounds, followed by wrapping the wound with Planchonia careya bark (Lassak and McCarthy, 1993). As well as protecting the wound, the bark itself was also thought to have antibacterial properties.

Much of the information about the antimicrobial activities of Australian plants is anecdotal. Few of the Aboriginal medicinal plants have been scientifically investigated for their antimicrobial activities. One study (Palombo and Semple, 2001) examined a panel of plant extracts commonly used by Australian Aboriginals and found approximately 20% of the samples tested were able to inhibit bacterial growth. This group has also demonstrated the antiviral activity of the same Australian plants (Semple et al., 1998). There are many other Australian plants, some used by Australian Aboriginals, that have not as yet been properly examined for antibacterial activity. This research was carried out to further extend these studies and screen a variety of other Australian plants for antimicrobial activity. The antimicrobial activities of thirty nine extracts from twenty five Australian plants against four bacteria is reported.

Materials and Methods

Plant Material

Test Microorganisms

Evaluation of Antimicrobial Activity

Results and Discussion

Thirty nine samples from twenty five Australian native plant species were extracted in methanol, dried and the weight of extracted material recorded (table 1). The weight of dried extractable material varied across samples, ranging from 83 mg (L. juniperium flowers) extracted per 1 g starting plant material up to 567 g (C. citrinus flowers) from the original 1 g of ground dried plant material. All extracts were resuspended in 15 ml of 20 % methanol, resulting in the crude test extract concentrations reported in table 1.

Figure 1

Table 1: Botanical names of plant species extracted, weight of dried extractable material and the concentrations of the extract used to determine antimicrobial activity.

Antimicrobial activity of the extracts was determined by disc diffusion assays. Twenty nine of the thirty nine extracts tested (72%) showed antibacterial activity against one or more bacteria. Indeed, of the twenty five plant species tested only 3 species had no inhibitory activity towards any of the bacteria tested in any of their extracts (D. pruriens var. jerseyana, S. leuhmannii, W. fruticosa). With respect to D.pruriens (fruit) and S. leuhmannii (leaves), only a single plant part was available for extraction and testing. Perhaps these plants may have also shown antibacterial activity if leaves and/or flowers were also tested. Two species (B. citriodora, and C. citrinus) were particularly versatile, being capable of inhibiting the growth of all four bacteria tested. These results were not surprising. The antibacterial activity of B. citriodora is well known (Dupont et al., 2006; Wilkinson et al., 2003) and recent studies have reported on the antibacterial activity of extracts from a different species of Callistemon (C. rigidus) (Sanjai and Charu, 2006).

Figure 2

Table 2: Antibacterial activity of plant extracts. Numbers indicate the mean diameters of inhibition of triplicate experiments ± standard deviation. – indicates no growth inhibition. NM indicates zone of inhibition not measured.

Gram-positive bacteria (B. cereus and B. subtilisis) were the most susceptible to growth inhibition by the plant extracts. The greater susceptibility of Gram-positive bacteria has been previously reported for South American (Paz et al., 1995), African (Kudi et al., 1999; Vlietinck et al., 1995) and Australian (Palombo and Semple, 2001) plant extracts. Susceptibility differences between Gram-positive and Gram-negative bacteria may be due to cell wall structural differences between these classes of bacteria. The Gram-negative bacterial cell wall outer membrane appears to act as a barrier to many substances including antibiotics (Tortora et al., 2001). B. cereus was especially susceptible, being inhibited by extracts from twenty one of the twenty nine plants tested (72%).

In contrast to the Palombo and Semple (2001) report, many of the Australian native plant extracts examined in the present report also had significant activity towards Gram-negative bacteria. These differences relate to the different species studied, but may also relate to the extract concentrations tested. Some of the extracts tested in this study were tested at concentrations as high as 30-40 mg extracted material per ml. No mention is made in the Palombo and Semple report (2001) of the concentrations of extracted material tested so the possibility exists that lower doses were used in those studies. However, the concentrations tested in this study were comparable to the 36 mg/ml extracts used by Kudi et al. (1999) and the 10mg/ml extracts tested by Paz et al. (1995) and are substantially lower than the doses tested (100 mg/0.2ml) by Vlietinck et al. (1995).

The Callistemon extracts tested exhibited the greatest antimicrobial activities (as determined by the diameters of the zones of inhibition) towards the most susceptible bacteria, B. cereus. These extracts also displayed good antibacterial activity towards Gram-negative bacteria, particularly P. flourescens. E. major and L. bracteata were also notable for their strong antibacterial activity against Gram-negative bacteria.

Noteworthy was the apparent trend that flower extracts generally were more potent inhibitors of bacterial growth than were leaf extracts from the same plant. In only one case (L. juniperium) were leaf extracts found to have better antimicrobial activities than flower extracts from the same plant. In all other cases, when both leaves and flowers were extracted, flower extracts were better antimicrobial agents. Unfortunately flower extracts were not able to be tested for all species to determine whether this trend holds up due to the unavailability of the flowers of some plants. Further studies may determine whether this trend also applies to other plants.

The results of this study provide further evidence of the antimicrobial activities of some Australian native plants. This study indicates that A. aulacocarpa, B. citriodora, B. celsissima, A. littoralis, as well as members of the Callistemon, Eucalyptus, Leptospermum and Syzygium genuses are particularly worthy of further study due to the range of bacteria they are capable of inhibiting. Further evaluation of the antibacterial properties of these extracts against a more extensive panel of microbial agents is warranted. Likewise, purification and identification of the bioactive components is needed to examine the mechanisms of action of these agents. Whilst the extracts examined in this report appear promising as antimicrobial agents, caution is needed before these compounds can be applied to medicinal purposes and as food additives to inhibit spoilage. In particular, toxicity studies are needed to determine the suitability of these extracts for these purposes.

Acknowledgements