For many years bacteria were considered primarily as autonomous unicellular organisms with little capacity for collective behaviour. However, we now appreciate that bacterial cells are in fact, highly communicative. The generic term 'quorum sensing' has been adopted to describe the bacterial cell-to-cell communication mechanisms which co-ordinate gene expression usually, but not always, when the population has reached a high cell density. Quorum sensing depends on the synthesis of small molecules (often referred to as pheromones or autoinducers) that diffuse in and out of bacterial cells. As the bacterial population density increases, so does the synthesis of quorum sensing signal molecules, and consequently, their concentration in the external environment rises. Once a critical threshold concentration has been reached, a target sensor kinase or response regulator is activated (or repressed) so facilitating the expression of quorum sensing-dependent genes. Quorum sensing enables a bacterial population to mount a co-operative response that improves access to nutrients or specific environmental niches, promotes collective defence against other competitor prokaryotes or eukaryotic defence mechanisms and facilitates survival through differentiation into morphological forms better able to combat environmental threats. Quorum sensing also crosses the prokaryotic-eukaryotic boundary since quorum sensing-dependent signalling can be exploited or inactivated by both plants and mammals.

SummaryIn Staphylococcus aureus, the agr locus is responsible for controlling virulence gene expression via quorum sensing. As the blockade of quorum sensing offers a novel strategy for attenuating infection, we sought to gain novel insights into the structure, activity and turnover of the secreted staphylococcal autoinducing peptide (AIP) signal molecules. A series of analogues (including the L-alanine and D-amino acid scanned peptides) was synthesized to determine the functionally critical residues within the S. aureus group I AIP. As a consequence, we established that (i) the group I AIP is inactivated in culture supernatants by the formation of the corresponding methionyl sulphoxide; and (ii) the group I AIP lactam analogue retains the capacity to activate agr, suggesting that covalent modification of the AgrC receptor is not a necessary prerequisite for agr activation. Although each of the D-amino acid scanned AIP analogues retained activity, replacement of the endocyclic amino acid residue (aspartate) located C-terminally to the central cysteine with alanine converted the group I AIP from an activator to a potent inhibitor. The screening of clinical S. aureus isolates for novel AIP groups revealed a variant that differed from the group I AIP by a single amino acid residue (aspartate to tyrosine) in the same position defined as critical by alanine scanning. Although this AIP inhibits group I S. aureus strains, the producer strains possess a functional agr locus dependent on the endogenous peptide and, as such, constitute a fourth S. aureus AIP pheromone group (group IV). The addition of exogenous synthetic AIPs to S. aureus inhibited the production of toxic shock syndrome toxin (TSST-1) and enterotoxin C3, confirming the potential of quorum-sensing blockade as a therapeutic strategy.

(Gut 2001;48:176-185)

Staphylococcus aureus is a versatile pathogen capable of causing life-threatening infections. Many of its cell wall and exoproduct virulence determinants are controlled via the accessory gene regulator (agr). Although considered primarily as an extracellular pathogen, it is now recognized that S. aureus can be internalized by epithelial and endothelial cells. Traditional experimental approaches to investigate bacterial internalization are extremely time-consuming and notoriously irreproducible. We present here a new reporter gene method to assess intracellular growth of S. aureus in MAC-T cells that utilizes a gfp-luxABCDE reporter operon under the control of the Bacillus megaterium xylA promoter, which in S. aureus is expressed in a growth-dependent manner. This facilitates assessment of the growth of internalized bacteria in a nondestructive assay. The dual gfplux reporter cassette was also evaluated as a reporter of agr expression and used to monitor the temporal induction of agr during the MAC-T internalization process. The data obtained suggest that agr induction occurs prior to endosomal lysis and that agr-regulated exoproteins appear to be required prior to the release and replication of S. aureus within the infected MAC-T cells.Staphylococcus aureus is the etiologic agent of numerous infections in humans and domesticated animals and has been implicated in a multitude of diseases, ranging from minor wound infections to more serious diseases, including endocarditis, osteomyelitis, and septic shock (reviewed by Projan and Novick [34]). The expression of many S. aureus virulence factors is under the control of the accessory gene regulator (agr) which, on entering post-exponential phase, downregulates the production of cell surface-associated proteins and upregulates the expression of secreted toxins and extracellular enzymes (28,33,38). The role of the agr regulon is supported by in vivo studies, which show that agr mutants are greatly attenuated in several animal models, including intramammary infections (13), arthritis in mice (1), and endocarditis in rabbits (7). The agr locus is a quorum-sensing-regulated system activated by autoinducing peptide pheromone (AIP) (21, 25). The agr locus consists of two divergent transcriptional units, RNAII and RNAIII, which are under the control of the P2 and P3 promoters, respectively (reviewed by Novick and Muir [30]). RNAII is a polycistronic mRNA that encodes the agrB and agrD genes required for the synthesis of the AIP and also the two component signal transduction proteins, AgrA and AgrC, which are responsible for sensing and responding to the AIP. RNAIII is the effector molecule in the agr regulon acting primarily at the level of gene transcription. Different S. aureus strains produce AIPs with distinct structures, and strains can be grouped on this basis since they will activate the agr response of strains within the same group and inhibit the agr response of strains from different groups by competitive inhibition (21,30). This inhibitory action of AIPs has identi...

The post-translationally modified peptide antibiotic nisin has been cleaved by a number of proteases and the fragments produced purified, characterised chemically, and assayed for activity in inhibiting the growth of Lactococcus lactis MG1614 and Micrococcus luteus NCDO8166. These results provide information on the importance of different parts of the nisin molecule for its growth-inhibition activity. Removal of the C-terminal five residues leads to approximately a 10-fold decrease in potency, while removal of a further nine residues, encompassing two of the lanthionine rings, leads to a 100-fold decrease. There are some differences between analogous fragments of nisin and subtilin, suggesting possible subtle differences in mode of action. Cleavage within, or removal of, lanthionine ring C essentially abolishes the activity of nisin. The fragment ulsin t-12 is inactive itself, and specifically antagonises the growth-inhibitory action of nisin. These results are discussed in lerms of current models for the mechanism of action of nisin.Key words: Nisin; Lantibiotic; Peptide antibiotic; I'roteolysis; Structure-activity relationships mation of voltage-dependent pores in biological membranes [8][9][10], although recent evidence indicates that the inhibition of the outgrowth of spores by nisin and subtilin takes place by a different mechanism [11,12]. We have been studying the structure-activity relationships in nisin and subtilin, by both genetic and chemical modification of the structure [12][13][14][15][16][17][18][19][20], with a view to understanding their mechanism of action and to developing new derivatives with desirable properties. We now report the preparation, characterisation and anti-bacterial activity of a number of proteolytic fragments of nisin and subtilin, which allow us to define the parts of the molecule most important for biological activity. Materials and methodsNisin and subtilin were prepared as previously described [14,15]; [Ser33]-nisin, in which the serine residue at position 33 has escaped processing to a dehydroalanine residue, was isolated as a minor component of commercial nisin. All proteases were obtained from Sigma Chemical Co., Poole, Dorset, UK. ~. IntroductionThe post-translationally modified peptide antibiotics known :ts 'lantibiotics' contain cyclic structures formed by lanthioaine or 3-methyl-lanthionine residues, and often also dehydroalanine and/or dehydrobutyrine residues [1]. The first lan-!ibiotic to be characterised was nisin, produced by strains of Lactococcus lactis carrying a transposon containing genes coding for the nisin precursor and for proteins involved in aisin biosynthesis and resistance [2][3][4][5]. Nisin has been quite videly used as a food preservative, notably in cheese and ,~ther dairy products and in canned vegetables, for some 30 .,ears [6,7]. It inhibits the growth of a wide range of Gram~ositive organisms, and also inhibits the germination and/or mtgrowth of spores of Bacillus and Clostridium species [6]. Fhe growth-inhibitory activity of nisi...