An Assay to Determine Polymicrobial Biofilm Initiation on Virus-Infected Cells

Published: March 29, 2024

Abstract

Source: Plotkin, B. J. et al., Determination of Biofilm Initiation on Virus-infected Cells by Bacteria and Fungi. J. Vis. Exp. (2016)

This video illustrates the initiation of polymicrobial biofilm formation on virus-infected cells by bacteria and fungi. Herpes simplex virus infection impedes bacterial attachment to the cells, preventing bacterial-fungal co-localization. In contrast, uninfected cells display bacterial-fungal co-localization and initiation of biofilm formation.

Protocol

1. HSV Strains and Handling

NOTE: Recombinant non-spreading HSV-1(KOS) gL86 and HSV-2 (KOS) 333gJ with beta-galactosidase reporter activity is used for the assay.

  1. Use virus from a single lot and store at -80 °C at a 1:1 ratio of Dulbecco's modified Eagle's medium (DMEM) with 20% fetal bovine serum (FBS) and skim milk until use. Before viral lot storage, determine virus concentration by o-nitrophenyl-β-D-galactopyranoside (ONPG) and 5-bromo-4-chloro-3-indolyl-β-D-galactopyranoside (X-Gal) assay.
  2. Determine virus viability and multiplicity of infection (MOI) by X-Gal staining for each experimental assay run using reporter virus entry assay, as previously described (Figure 1).
  3. Dilute virus (Opt-MEM) to desired MOI. Fix monolayers (paraformaldehyde; 0.5 ml/well) before staining. Place virus viability controls in a separate microtiter plate in parallel with polymicrobial assay plates.

2. HeLa 299 Cell Handling

  1. Grow at 37 °C, 5% COin DMEM with 4.5 g/L glucose, 10% heat-inactivated fetal bovine serum (FBS), gentamicin (50 µg/ml) and L-glutamine. Passage cells at 80% confluence with Trypsin solution (ethylenediaminetetraacetic acid, 0.53 M EDTA; 0.05% Trypsin; 5 ml/flask).

3. C. albicans Handling

NOTE: C. albicans obtained from a clinical laboratory source is stored at -80 °C in Remmel skim milk 2x medium.

  1. Culture frozen stock onto Sabouraud Dextrose Medium (37 °C). After 24 hr subculture the C. albicans onto Fungisel medium (37 °C; 48 hr) for use.
  2. Generate germ tube (GT) forms (pick representative colonies; 3 ml FBS; 3 hr; 37 °C; absorbance600 (Abs600), 0.3). After incubation, wash GT (Hanks' Balanced Salt Solution, HBSS; 2x; 4,000 x g). Add washed GT to warmed HBSS (37 °C; 0.32 Abs600). GT forms should be 99% of cells observed as determined by hemocytometer count.
  3. Make yeast form (YF) stock suspensions by picking representative Fungisel colonies (HBSS, 3 ml; 0.32 Abs600). Count the number of YF forms/ml microscopically using a hemocytometer. YF forms should be 99% of cells observed as determined by hemocytometer count.
  4. Make working fungal stock (250 µl of GT or YF stock in 25 ml HBSS; 37 °C; 105 colony forming unit, (CFU/ml)

4. S. aureus Handling

  1. Store S. aureus American Type Culture Collection, ATCC 25923 (-80 °C; Remmel skim milk 2x). Culture onto sheep blood agar (5%; 37 °C; 24 hr). Pick representative colonies and transfer to mannitol salts medium within 2 days for stock (37 °C; 18 hr).
  2. Make S. aureus stock suspension (3 ml HBSS; 1.32 Abs600 ;108 CFU/ml)
    1. Make working S. aureus stock (100 µl of the stock in 25 ml HBSS; 105 CFU/ml).

5. Candida and S. aureus Suspensions

  1. Make mixed C. albicans and S. aureus suspension (250 µl YF or GT stock and 100 µl S. aureus stock in 25 ml HBSS).

6. Polymicrobial Biofilm Assay

  1. Seed 96 well plates with 200 µl of 2 x 105 HeLa cells/ml (85% confluence level). Rock plates (30 – 45 min; 37 °C) before incubation (37 °C; 5% CO2 incubator; 18 hr). Wash monolayers (1x; Opt-MEM) then seed with HSV (HSV-1 (KOS) gL86 or HSV-2 (KOS) 33 gJ in 100 µl Opt-MEM ; MOI 50 and 10). Incubate plates (3 hr; 37 °C; 5% CO2). Use only one viral strain per day.
  2. Wash infected monolayers (1x; phosphate-buffered saline (PBS) with Mg+2 and Ca+2; 100 µl). Replace PBS with warm HBSS leaving 25 µl in each well.
    1. Add YF, GT and/or S. aureus working suspensions (100 µl; target to cell ratio =5:1; n=16) as indicated in Table 1. Incubate plates (static; 30 min; 37 °C; 5% CO2).
  3. After incubation, aspirate one column at a time immediately refilling with 300 µl PBS with Mg+2 and Ca+2. Repeat this step twice then add radio-immunoprecipitation assay lysis buffer (RIPA; filter sterilized; 200 µl of a 1:50 dilution).
  4. Rapidly triturate the HeLa cell lysate then place 50 µl onto mannitol salts (MS) and/or Fungisel (F) media (Figure 2). Spread the lysate using a glass rod bent at a 90° angle. Incubate the plates (18 hr at 37 °C). Manually count the number of colonies per plate. Controls consist of S. aureus and/or C. albicans adherence to HSV-uninfected HeLa cells.

Equation1

Table 1. General Template Determination of Microbial Adherence in Polymicrobial Interactions. GT= Candida albicans germ tube phenotype; YF= C. albicans yeast form phenotype; MSSA= methicillin-sensitive Staphylococcus aureus; MS= mannitol salts medium; F= Fungisel medium.

Representative Results

Figure 1
Figure 1. X-gal Staining Pictures of Dosage Dependent HSV-1 Infection in HeLa Cells. HeLa cells infected with HSV-1 at various MOI and X-Gal stained. (A) HeLa cells in well of 96 well plate with X-Gal stained mock-infected HeLa cell control; 20x initial magnification; (B) HeLa cells in well of 96 well plate with X-Gal stained HeLa cell infected with HSV-1 at an multiplicity of infection (MOI) of 10; 20x initial magnification; (C) HeLa cells in well of 96 well plate with X-Gal stained HeLa cell infected with HSV-1 at an multiplicity of infection (MOI) of 50; 20x initial magnification; scale bar applies to all.

Figure 2
Figure 2Effect of HSV-1 (panels A, C, E) and HSV-2 (panels B, D, F) at Multiplicities of Infection (MOI) of 50 and 10 on Adherence of S. aureus and/or C. albicans to HeLa Cells. (AS. aureus (Sa) binding to HSV-1 infected cells in the presence of C. albicans germ tubes (GT) or yeast forms (YF); (B) S. aureus (Sa) binding to HSV-2 infected cells in the presence of C. albicans germ tubes (GT) or yeast forms (YF); (CC. albicans germ tubes (GT) binding to HSV-1 infected cells in the presence of S. aureus (Sa)(DC. albicans germ tubes (GT) binding to HSV-2 infected cells in the presence of S. aureus (Sa); (EC. albicans yeast forms (YF) binding to HSV-1 infected cells in the presence of S. aureus (Sa); (F) C. albicans yeast forms (YF) binding to HSV-2 infected cells in the presence of S. aureus (Sa). All data points are Mean +/- SEM, n= 16 normalized to virus-free control. * = significantly different (p< 0.05) from uninfected HeLa cell control. # = significantly different (p< 0.05) from paired point indicated by bracket; scale bar applies to all.

Offenlegungen

The authors have nothing to disclose.

Materials

C.albicans
BBL Sabouraud Dextrose BD 211584
Fungisel Agar Dot Scientific 7205A
S.aureus
Mannitol Salt Agar Troy Biologicals 7143B
Sheep blood agar Troy Biologicals 221239
Hela cells
1xDMEM (Dubelcco's Modified Eagle Medium, with 4.5 g/L glucose and L-glutamine, without sodium pyruvate Corning 10-017-CM
Gentamicin 50mg/ml Sigma 1397 50µg/ml final concentration in the complete DMEM
Trypsin EDTA (0.05% Trypsin, 0.53M EDTA)Solution 1X Corning 25-052-CI
Fetal Bovine Serum Atlanta Biologicals S11150 10% final concentration in the complete DMEM
Other medium and reagents
ONPG Thermo Scientific 34055
Ultra-Pure X gal Invitrogen 15520-018
1x HBSS (Hanks' Balanced Salt Solution) Corning 20-021-CV
1XPBS Dot Scientific 30042-500
RIPA Lysis Life Technologies 89901
Supplies
96-Well plates Evergreen Scientific 222-8030-01F
Culture tubes 100×13 Thomas Scientific 9187L61

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An Assay to Determine Polymicrobial Biofilm Initiation on Virus-Infected Cells. J. Vis. Exp. (Pending Publication), e22008, doi: (2024).

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