Antimicrobial Susceptibility Assay and Kit

Abstract

The invention relates to a method for rapidly determining the susceptibility of a microorganism to an antimicrobial agent comprising the steps: a) contacting a first sample containing the microorganism with a first growth medium so as to form a first mixture, wherein the first growth medium is selected to enable the microorganism to proliferate and/or encourage the microorganism cell cycle to commence proliferation; b) contacting a second sample containing the microorganism with a second growth medium so as to form a second mixture, wherein the second growth medium is substantially the same as the first growth medium but further comprises a first antimicrobial agent which may inhibit or slow the proliferation of the microorganism; c) incubating the first and second mixtures, for 30 minutes or less, under conditions suitable to enable or encourage proliferation of the microorganism; d) passing the first and second mixture, or portion thereof, through a flow cytometer in order to assess one or more biochemical and/or biophysical parameters of the microorganisms in both mixtures; and e) comparing the parameters of the microorganisms in the first mixture with that of the second mixture, after incubation, in order to detect whether the first antimicrobial agent inhibits or slows the proliferation of the microorganism so as to determine the susceptibility of a microorganism to said agent. The method is particularly suited for identifying the which antimicrobial agents would be suitable for the treatment of microbial infections, such as Urinary Tract Infections (UTIs)

Claims

1. A method for rapidly determining the susceptibility of a microorganism to an antimicrobial agent comprising the steps: a) contacting a first sample containing the microorganism with a first growth medium so as to form a first mixture, wherein the first growth medium is selected to enable the microorganism to proliferate and/or encourage the microorganism cell cycle to commence proliferation; b) contacting a second sample containing the microorganism with a second growth medium so as to form a second mixture, wherein the second growth medium is substantially the same as the first growth medium but further comprises a first antimicrobial agent which may inhibit or slow the proliferation of the microorganism; c) incubating the first and second mixtures, for 30 minutes or less, under conditions suitable to enable or encourage proliferation of the microorganism; d) passing the first and second mixture, or portion thereof, through a flow cytometer in order to assess one or more biochemical and/or biophysical parameters of the microorganisms in both mixtures; and e) comparing the parameters of the microorganisms in the first mixture with that of the second mixture, after incubation, in order to detect whether the first antimicrobial agent inhibits or slows the proliferation of the microorganism so as to determine the susceptibility of a microorganism to said agent.

2. The method as claimed in claim 1, wherein step b) further comprises contacting one or more further samples containing the microorganism with a one or more further growth media so as to form one or more further mixtures, wherein the one or more further growth media is the same as the first growth medium but further comprises one or more further antimicrobial agents which may inhibit or slow the proliferation of the microorganism, wherein said one or more further antimicrobial agents are different from one another and different from the first antimicrobial agent.

3. The method as claimed in any proceeding claim, wherein the one or more biochemical and/or biophysical parameters of the microorganisms is selected from one or more of the following: cell size, cell number, cell membrane energisation and/or nucleic acid content and/or distribution.

4. The method as claimed in any preceding claim, wherein the one or more biochemical and/or biophysical parameters of the microorganisms is determined by assessing the uptake of one of more fluorescent or other stains.

5. The method as claimed in any one preceding claim, wherein the parameter is cell size, cell number and/or cell membrane energisation, and wherein the medium further comprises a carbocyanine dye or prior to step d), carbocyanine dye is added to the mixture or part of the mixture.

6. The method as claimed in claim 5, wherein the carbocyanine dye comprises 3,3′-dipropylthiadicarbocyanine iodide (di-S-C3(5)).

7. The method as claimed in either claim 5 or 6, wherein the carbocyanine dye is present in the mixtures at a concentrate in the range of about 1 μM to about 5 μM.

8. The method as claimed in claim 7, wherein the carbocyanine dye is present in the mixtures at a concentrate in the range of about 3 μM.

9. The method as claimed in any one of claims 5 to 8, wherein the flow cytometer relies upon excitation at 640 nm and the parameters are assessed at 675±15 nm.

10. The method as claimed in any preceding claim, wherein the parameter is nucleic acid and said nucleic acid comprises DNA.

11. The method as claimed in claim 9, wherein prior to step d), mithramycin and/or a nucleic acid stain are added to the mixture or part of the mixture, and optionally, DNA distribution is assessed on the flow cytometer at around 572 nm.

12. The method as claimed in claim 11, wherein the nucleic acid strain comprises SYBR Green or ethidium bromide.

13. The method as claimed in any preceding claim, wherein the growth medium comprises Terrific Broth.

14. The method as claimed in any preceding claim, wherein step c) takes place at a temperature in the range of about 35° C. and 40° C.

15. The method as claimed in 14, wherein step c) takes place at a temperature of about 37° C.

16. The method as claim in any preceding claim, wherein a portion of the first and second mixture, or portion of the one or more further mixtures when dependent upon claim 2, is assessed at multiple time points.

17. The method as claimed in claim 16, wherein the multiple time points comprise one or more of the following time points, 0 minutes, 5 minutes, 10 minutes, 15 minutes, 20 minutes, 25 minutes and/or 30 minutes.

18. The method as claimed in any one of claims 1 to 15, wherein step c) is about 20 minutes or less.

19. The method as claimed in any one of claims 1 to 15, wherein step c) is in the range of about 15 minutes to about 20 minutes.

20. The method as claimed in any preceding claim, wherein step d) is conducted prior to, and after step c).

21. The method as claimed in any preceding claim, wherein the microorganism is obtained from a biological sample derived from an individual believed to be suffering from a microorganism infection.

22. The method of claim 21, wherein the biological sample is urine.

23. The method as claimed in claim 21, wherein the microorganism infection is a Urinary Tract Infection (UTI).

24. The method as claimed in any preceding claim, for determining the antimicrobial agent for use in the treatment of a microorganism infection in an individual, wherein the method comprises taking a biological sample from the individual, assessing the susceptibility of the microorganism, in the biological sample, to two or more antimicrobial agents and identifying which antimicrobial agent to administer to the individual based which antimicrobial agent inhibits or slows the proliferation of the microorganism.

25. A kit for rapidly determining the susceptibility of a microorganism to an antimicrobial agent comprising: a) an enriched growth medium; b) one or more antimicrobial agents; and c) a carbocyanine dye.

26. The kit as claimed in claim 25, wherein the kit further comprises: d) a flow cytometer.

27. The kit as claimed in claim 25 or 26, wherein the enriched growth medium comprises Terrific Broth.

28. The kit as claimed in claim 26, wherein the flow cytometer comprises at least one red laser.

29. The kit as claimed in any of claim 25 or 28, wherein the kit is for use in the method as claimed in any one of claims 1 to 24.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0041] Embodiments of the invention are described below, by way of example only, with reference to the accompanying figures in which:

[0042] FIG. 1. True and apparent lag phases during microbial regrowth. The strains indicated were grown in Lysogeny Broth and inoculated into Terrific Broth after 4 h in stationary phase to ca 10.sup.5 cells.Math.mL.sup.−1. OD was measured quasi-continuously in an Omega plate reader spectrophotometer (BMG Labtech, UK), while CFU were measured conventionally on agar plates containing nutrient agar medium solidified with 1.5% agar. The lag phase observed via counting CFUs is less than 30 min while bulk OD measurements show a lag phase of some 230 minutes (˜4 hours).

[0043] FIG. 2. Cytograms of E. coli at a concentration of 10.sup.5 cells.Math.mL.sup.−1 when incubated in 0.2 μm-filtered Terrific Broth containing 3 μM Di-S-C3(5). The sample measured has a volume of 3 μL and measurement takes place over 2 seconds. A. 1D histograms of RL1 fluorescence showing the reproducibility of the results. B. 1D histograms of RL1 fluorescence together with calibrating beads, showing that the breadth of the bacterial peaks is ‘real’ and not simply due to detector variability. C and D. Raw dot plots of the height of the forward scatter and of side scatter signals respectively vs RL1. Note that the E. coli cells appear above a value of 10.sup.5 in RL1, the rest of the signals being due to very tiny unfiltered debris. E. Gating strategy (I-V) to show only the E. coli singlet cells.

[0044] FIG. 3. Changes in cell number during first 30 min following inoculation of cells from stationary phase into Terrific Broth. A. Typical cytograms. B. Reproducibility and Z′ statistics (see below) for E. coli growth at initial concentration of 10.sup.5 cells.Math.mL.sup.−1. C. Reproducibility and Z′ statistics for E. coli inoculated at 5×10.sup.5 cells.Math.mL.sup.−1.

[0045] FIG. 4. Effect of ampicillin (100 mg.Math.L.sup.−1 concentration, 3×MIC for sensitive strains) on the cytograms of E. coli inoculated from stationary phase into Terrific Broth. Ampillicin was either absent (A,C) or present (B,D) from resistant strain 16 (A,B) or sensitive strain 7 (C,D). E. Table showing the changes in the number of bacteria (with replicates) from sensitive (strain 7) and resistant strains (strain 16) when grown in the presence and absence of Ampicillin. Similar data were obtained using eight other macroscopically sensitive and resistant strains.

[0046] FIG. 5. Side scatter histograms of the experiment mentioned in FIG. 4. Ampillicin was either absent (A,C) or present (B,D) from resistant strain 16 (A,B) or sensitive strain 7 (C,D).

[0047] FIG. 6. Effect of nitrofurantoin at 3× nominal MIC on the growth and flow cytometric behaviour of a sensitive strain of E. coli. A,B for nitrofurantoin, cytograms of (A) side scatter, (B) RL1 fluorescence. Experiments were performed precisely as shown in the legend to FIG. 3. (C) Ability of flow cytometric particle counting (gated as in FIG. 2) to determine the sensitivity of E. coli MG1655 to four different antibiotics in 20 mins.

[0048] FIG. 7. DNA distributions in different populations. (A). DNA distributions in stationary phase (red) and exponentially growing cells (blue). The overlay histogram shows data from E. coli samples that were fixed with 70% ice cold ethanol and then stained using Mithramycin and Ethidium Bromide as described in Materials and Methods. The relative intensity of the BL2 channel fluorescence (488 nm excitation, 572±14 nm emission) shows the amount of chromosomes in the cells. The points I, II and III represent one, two and eight chromosome equivalents, respectively. The peak values of BL2 fluorescence for the points are I (2.03.10.sup.4), II (3.90.10.sup.4) and III (1.53.10.sup.5). The cells in stationary phase (2-4 h) were taken and fixed immediately while the exponentially growing cells were incubated for 90 minutes at 37° C. before fixing the cells. (B). Changes in DNA distribution in E. coli cells following inoculation from a stationary phase into Terrific Broth every 5 min until 30 min. Experiments were otherwise performed exactly as described in the legend to in FIG. 2. except that (to avoid spectral interference) carbocyanine was not present.

[0049] FIG. 8. Cytograms of a sensitive UTI strain treated with nitrofurantoin. UTI samples (in this case containing ˜10.sup.6 cells.Math.mL.sup.−1) were taken directly from storage, diluted tenfold into 37° C. Terrific Broth including 3 μM diS-C3(5) and nitrofurantoin at a nominal 3×MIC, and measured flow cytometrically as described in the legend to FIG. 2. (A) side scatter, (B) red fluorescence.

[0050] MATERIALS AND METHODS

[0051] Microbial Strains.

[0052] E. coli MG1655 and a series of sensitive and resistant strains were taken from a laboratory collection.

[0053] Culture

[0054] E. coli strains were grown from inocula of appropriate concentrations in conical flasks using Lysogeny Broth to an optical density (600 nm) of 1.5-2, representing stationary phase in this medium. They were held in stationary phase for 2-4 h before being inoculated at concentration of 10.sup.5 cells.Math.mL.sup.−1 (or as noted) into Terrific Broth (Tartof and Hobbs 1987). We did not here study cells held in a long stationary phase (Finkel 2006; Navarro Llorens et al. 2010) (exceeding 3d).

[0055] Assessment of Growth by Bulk OD Measurements

[0056] Bulk OD measurements were performed in 96-well plates and read at 600 nm as per the manufacturer's instructions in an Omega plate reader spectrophotometer (BMG Labtech, UK) instrument. The ‘background’ due to scattering from the plates, etc., was not subtracted.

[0057] Flow Cytometry

[0058] Initial studies used a Sony SH-800 instrument, but all studies reported here used an Intellicyt® iQue screener PLUS. This instrument is based in significant measure on developments by Sklar and colleagues (e.g. (Edwards et al. 2009; Sklar et al. 2007; Tegos et al. 2014)), and uses segmented flow (Skeggs 1957) to sample from 96- or 384-well U- or V-bottom plates prior to their analysis. The iQue Plus contains three excitation sources (405 nm, 488 nm, 640 nm) and 7 fixed filter detectors (with a midpoint/range in nm of 445/45, 530/30, 572/28, 585/40, 615/24, 675/30, 780/60, giving 13 fluorescence channels) whose outputs are stored as both ‘height’ and area, using the FCS3.0 data file standard (Seamer et al. 1997). Forward and side scatter are obtained from the 488 nm excitation source. Detection channels are referred to by the laser used (405 nm violet VL, 488 nm blue BL, and 640 nm red RL) and the detector number in order of possible detectors with a longer wavelength. Thus RL1, as used for detecting di-S-C3(5), implies the red laser and the 675/30 detector. Data are collected from all channels, using a dynamic range of 7 logs. Many parameters may be used to vary the precise performance of the instrument. Those we found material to provide the best reproducibility and to minimise carryover, and their selected values, are as follows: Automatic prime—60 secs (in Qsol buffer); Pre-plate shake—15 s and 1500 rpm; Sip time—2 s (actual sample uptake); Additional sip time—0.5 s (the gap between sips); Pump speed—29 rpm (1.5 μL.Math.s.sup.−1 sample uptake); Plate model—U-bottom well plate (for 96 well plates); Mid plate cleanup—After every well (4 washes; 0.5 s each in Qsol buffer); Inter-well shake—1500 rpm; after 6 wells, 4 sec in Qsol buffer; Flush and Clean—30 sec with Decon and Clean buffers followed by 60 sec with deionised water. The Forecyt™ software supplied with the instrument may be used to gate and display all the analyses post hoc. It, and the FlowJo software, were used in the preparation of the cytograms shown. Where used, di-S-C3(5) was present at a final concentration of 3 μM; its analysis used excitation at 640 nm and detection at 675±15 nm, the fluorescence channel being referred to as RL1. For DNA analyses, cells were fixed by injection into ice-cold ethanol (final concentration 70%), washed twice by centrifugation in 0.1 M-Tris/HCl buffer, pH 7.4, before resuspension in the same buffer containing mithramycin (50 μg mL.sup.−1) and ethidium bromide (25 μg mL.sup.−1), MgCl.sub.2, (25 mM) and NaCl (100 mM) (Boye et al. 1983). Under these circumstances, the excitation energy absorbed by mithramycin (excitation 405 or 488 nm) is transferred to the ethidium bromide, providing a large Stokes shift (emission at 572, 585 or 615 nm; we chose 572 nm as it provided the best signal) and high selectivity for DNA (as mithramycin does not bind to RNA). All the solutions and media used were filtered through 0.2 μm filter.

[0059] UTI Samples

[0060] Following ethics approval from the University of Manchester and the obtaining of signed consent forms, patients attending the Firsway clinic with suspected UTI were offered to opportunity to have their urine samples analysed by our method as well as the reference method used in a centralised pathology laboratory. Samples were taken at various times during the day, kept at 4° C., delivered to the Manchester laboratory by taxi, plated out (LB agar containing as appropriate the stated antibiotics at 3 times normal MIC) to assess microbial numbers and antibiotic sensitivity, the remaining sample kept again at 4° C., and analysed flow cytometrically within 18 h. For flow cytometric assessment, cells were diluted into 37° C. Terrific Broth containing 3 μM diS-C3(5) plus any appropriate antibiotic, and assayed as above. For other experiments (not shown) cells were filtered (0.45 μm) and diluted as appropriate into warmed terrific broth. No significant differences were discernible in the two methods.

[0061] Reagents

[0062] All reagents were of analytical grade where available. Flow cytometric dyes were obtained from AAT Bioquest.

[0063] Results

[0064] Initial Assessment of Regrowth by Bulk Light Scattering Measurements in 96-Well Plates

[0065] FIG. 1 shows a typical lag phase from an inoculum of 10.sup.5 cells.Math.mL.sup.−1 that had spent 4 h in stationary phase when inoculated into Terrific Broth (Tartof and Hobbs 1987) as observed by bulk OD measurements. For strain MG1655 it amounts to some 230 min, while it is lower (2.5-3 h) for the more virulent clinical isolates (not shown). A rule of thumb states that an OD of 1 is approximately equal to 0.5 mg.Math.mL.sup.−1 dry weight bacteria or ˜10.sup.9.Math.cells.Math.mL.sup.−1 for E. coli. Thus, the change in OD if 10.sup.5 cells.Math.mL.sup.−1 increase their number by 50% is ˜0.00015, which is immeasurably small in this instrument. Given the noise in the system (probably mainly due to fluctuations in the incident light intensity), it is reasonable that we might, in this system, detect changes in OD of 0.01 (˜10.sup.7 cells.Math.mL.sup.−1), which requires a 100-fold increase in cell number over the inoculum (˜7 doublings). With a true lag phase of 10-15 min, and a doubling time of 20 min, this is indeed roughly what can be observed (FIG. 1)(see also (Chandra and Singh 2018; Pin and Baranyi 2006, 2008)). When samples were taken from the same strain and plated out to estimate proliferation by CFU, the results were indeed equally consistent with those at the longer times (FIG. 1).

[0066] Flow Cytometric Assessment of Cells and Cell Proliferation

[0067] FIG. 2A shows a typical set of traces of multiple wells from the Intellicyt iQue, each containing an inoculum of 10.sup.5 cells.Math.mL.sup.−1. Each analysis is of 3 μL (taking 2 s), and the good reproducibility is evident, especially in the inset stacked plot. We also show (rear trace) the cytograms of a bead cocktail; this FIG. 2B shows that the distribution in cell properties is significantly greater than that of beads, and its significant width is thus not due to any inadequacies in the detector. The quality of a ‘high-throughput’ (or indeed any other) assay is nowadays widely assessed using the Z′ statistic (Zhang et al. 1999). This is given, for an assay in which the sample's readout exceeds that of the control, as:

Z′=1−3(SD Sample+SD control)/(mean of sample−mean of control)  Eq. 1.

[0068] It is normally considered (Zhang et al. 1999) that a Z′ factor exceeding 0.5 provides for a satisfactory assay.

[0069] FIGS. 2C and 2D show the full cytograms for forward scatter and side scatter, respectively vs RL1, illustrating the amount of small particulates remaining in terrific broth, despite extensive filtering. Consequently, we used a series of gates to assess solely the bacteria in our samples. These are shown in FIG. 2E.

[0070] FIG. 3 shows cytograms at various times after inoculation of the stationary phase (LB-grown) cells into Terrific Broth, along with labels of cell numbers within the regions of interest. These allow the assessment of the Z′ values as per equation 1. From FIG. 3B it may be observed that Z′>0.5 from as early as 20 min, this then representing the earliest that we can robustly detect proliferation. Changes in cell constitution as judged by light scatter can, however, be detected from the earliest time point (5 min, FIG. 3A top left). It is noteworthy that the proliferation (as measured by the increase in cell numbers on the ordinate) is paralleled, at least initially, by an increase in uptake of the carbocyanine dye (on the abscissa); as the cells ‘wake up’ they become increasingly energised, until they settle down (also observed via side scatter). For a lower concentration of starting inoculum (5×10.sup.4 cells.Math.mL.sup.−1), the Z′>0.5 from 25 min as shown in FIG. 3C.

[0071] Flow Cytometric Assessment of Antibiotic Sensitivity

[0072] FIG. 4 shows similar data for a resistant (FIG. 4A,B) and a sensitive strain (FIG. 4C,D) in the absence ((4A,C) and presence (FIG. 4B,D) of the antibiotic ampicillin, applied at three times the known MIC (MIC=32 mg.Math.L.sup.−1)) (http://www.eucast.org/fileadmin/src/media.PDFs/EUCAST_files/Breakpoint_tables/v_8.1_Breakpoint_Tables.pdf) (The European Committee on Antimicrobial Susceptibility Testing. Breakpoint tables for interpretation of MICs and zone diameters. Version 8.1, 2018. http://www.eucast.org.). It is clear that the susceptible strain differs (and thereby can be discriminated) from the resistant strain in at least three ways: (i) the kinetics of changes in cell numbers as judged by RL1 counts, (ii) the same as judged by forward (not shown) or side scatter, (iii) kinetic changes in the magnitude of the fluorescence.

[0073] Since we had seen rapid changes in side scatter within 5 min (FIG. 3) it was also of interest to study this as a means of detecting antibiotic sensitivity. FIG. 5 shows that the changes in side scatter also differs noticeably between sensitive and resistant strains in 5-10 minutes, albeit that limited proliferation was taking place.

[0074] Of course different antibiotics have different modes of action (Brochado et al. 2018; Zampieri et al. 2018), and the optimal readout needs to reflect this. Thus, nitrofurantoin is widely prescribed for UTIs and its effects on our standard laboratory system are shown in FIG. 6A,B (cytograms of side scatter and RL1, respectively). The effects on cell proliferation of nitrofurantoin and several other antibiotics are given in FIG. 6C. Note that the initial and later cell numbers for nitrofurantoin appear lower because this antibiotic absorbs light at the excitation wavelength (its peak is at 620 nm). Both the bacteriostatic (trimethoprim) and bactericidal (ampicillin, ciprofloxacin, nitrofurantoin) antibiotics can be seen to work effectively on this sensitive strain.

[0075] Flow Cytometric Assessment of DNA Distributions

[0076] Another important strategy for detecting bacteria uses their DNA (e.g. (Hammes and Egli 2010; Jernaes and Steen 1994; Müller and Nebe-von-Caron 2010)). Thus, another high-level guide to the physiology of E. coli cells and cultures is the flow cytometrically observable distribution of DNA therein, as this can vary widely as a function of growth substrate, temperature, and during the cell cycle (Boye and Løbner-Olesen 1991; Skarstad et al. 1986; Skarstad et al. 1985; Steen and Boye 1980; Stokke et al. 2012). Specifically, the solution to the problem that DNA replication rates are fixed while growth rates can both vary and exceed them is to allow multiple replication forks in a given cell (Cooper and Helmstetter 1968). To this end, we compared the DNA distributions of our cultures under various conditions. FIG. 7A shows both stationary phase and exponentially growing cells stained with a mithramycin-ethidium bromide cocktail as per the protocol of Skarstad and colleagues given in Materials and Methods. As they have previously observed (Boye et al. 1983; Skarstad et al. 1985), (very slowly growing or) stationary phase cells display either one or two chromosome complements, while those growing exponentially in lysogeny broth (LB medium) can have as many as eight or more chromosomes. This is entirely consistent with the basic and classical Cooper-Helmstetter model (Cooper and Helmstetter 1968) and more modern refinements (Sauls et al. 2016; Si et al. 2017; Willis and Huang 2017; Zheng et al. 2016). To this end, FIG. 7B shows changes in the DNA distribution of cells taken from a similar regrowth experiment to that in FIG. 2. It is evident that both the one- and two-chromosome-containing cells from the stationary phase initiate increases in their DNA content on the same kinds of timescale as may be observed from both direct cell counting (proliferation) and carbocyanine fluorescence, with the initially bimodal DNA distribution morphing into a more monomodal one. This implies that the initial increase in cell numbers over 15 min or so involves cells that were about to divide actually dividing, and provides another useful metric of cellular (cell cycling) activity, albeit one that requires sampling as the cells must be permeabilised, at least for this protocol.

[0077] Flow Cytometric Analysis of UTI Samples

[0078] Finally, we wished to determine whether this method, as developed in laboratory cultures, could be applied to candidate UTI specimens ‘as received’ in a doctor's surgery. To this end, we analysed 23 samples, of which six were in fact positive as judged by a reference method performed in a central microbiology laboratory. Each of these was also found to be positive using our methods, and with the antibiotic sensitivities given in Table 1 below. These were again consistent with the reference method.

TABLE-US-00001 TABLE 1 Antibiotic sensitivity profile for the six positive samples (taken to be ≥10.sup.5 .Math. mL.sup.−1) obtained from the Firsway clinic. Antibiotic sensitivity (R—resistant; S—sensitive) Sample Date Ampicillin Trimethoprim Ciprofloxacin Nitrofurantoin May 25, 2018 R R S S May 25, 2018 S S S S Jun. 6, 2018 R S S S Jul. 16, 2018 R S S S Jul. 18, 2018 R S R R Jul. 20, 2018 S R S S

[0079] Typical cytograms for sensitive and resistant strains are given in FIG. 8. The positive cultures were speciated centrally, and in each case the organism was found to be E. coli.

DISCUSSION AND CONCLUSIONS

[0080] It is often considered that the lag′ phase of bacterial growth is one in which very little is happening, and that what is happening is happening quite slowly. This notion probably stems from the fact that changes in OD observable by the naked eye in laboratory cultures (Kaprelyants and Kell 1993) are indeed quite sluggish. However, the very few papers that have studied this in any detail (Baltekin et al. 2017; Madar et al. 2013; Novotna et al. 2003; Pin et al. 2009; Rolfe et al. 2012; Roostalu et al. 2008; Schoepp et al. 2017; Yu et al. 2018) have found that changes in expression profiles (albeit mainly measured at a bulk level) actually occur on a very rapid timescale indeed, possibly in 4 minutes or less following reinoculation into a rich growth medium. For antibiotics to have an observable, and in terms of sensitivity to them a differentially observable, effect on cells, the cells need to be in a replicative state. This might be thought to preclude any such observations in the lag phase, but what is clear from the present observations is that cells can re-initiate or continue their cell cycles very rapidly, such that observable proliferation can occur in as little as 15-20 min after reinoculation of starved, stationary phase cells into rich medium. Consequently it is not necessary to wait for a full period of ‘lag-plus-first-division time’ (Baltekin et al. 2017), which can be well over one hour (Pin and Baranyi 2006, 2008). The rapid proliferation that we describe could be observed by light scattering, by cell counting, by carbocyanine fluorescence (membrane energisation), and by changes in the magnitude and distribution of DNA in the population. This has allowed us to determine, using any or all of these phenotypic assays, antibiotic susceptibility at a phenotypic level in what would appear to be a record time. Pin and Baranyi (Pin and Baranyi 2006, 2008) observed a more stochastic and somewhat slower process than that which we observed here, but in their case they were measuring CFU only, and the inoculation was into the less rich LB, while we used Terrific Broth. Indeed, the exit from lag phase can be very heterogeneous when organisms are measured individually (Aguirre et al. 2013; Aguirre and Koutsoumanis 2016; Baltekin et al. 2017; Stylianidou et al. 2016).

[0081] While we did not study this at the level of the transcriptome here, the dynamics of the physiological changes observed during the early lag and regrowth phases as observed by the uptake of the carbocyanine dye are of interest. Classically, its uptake has been considered to reflect a transmembrane potential difference (negative inside) (e.g. (Bashford 1981; Ghazi et al. 1981; Johnson et al. 1981; Shapiro 2000; Waggoner 1976; Waggoner 1979), but cf. (Felle et al. 1978)) based on bilayer-mediated equilibration according to the Nernst equation (Rottenberg 1979). However, we recognise that such cyanine dyes, much as ethidium bromide (Jernaes and Steen 1994) and other xenobiotics (Kell et al. 2013; Kell and Oliver 2014), are likely to be both influx and efflux substrates for various transporters (Wu et al. 2015), so such an interpretation should be treated with some caution.

[0082] A similar strategy may usefully be applied to other cells (including pathogens in more difficult matrices such as urine), other antibiotics and other stains. However, the present work provides a very useful springboard for these by showing that one may indeed expect to be able to determine antibiotic susceptibility in a phenotypic assay in 20 minutes or less. This could be a very useful attribute in the fight against anti-microbial resistance.

[0083] The forgoing embodiments are not intended to limit the scope of the protection afforded by the claims, but rather to describe examples of how the invention may be put into practice.

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